Geologic history of an ash-flow sequence and its source area in the Basin and Range province of southeastern Arizona

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Authors Marjaniemi, Darwin Keith, 1940-

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/558744 GEOLOGIC HISTORY OF AN ASH-FLOW SEQUENCE AND

ITS SOURCE AREA IN THE BASIN AND RANGE PROVINCE

OF SOUTHEASTERN ARIZONA

by

Darwin Keith M arjaniemi

A Dissertation Submitted to the Faculty of the

DEPARTMENT OF GEOLOGY

In Partial Fulfillment of the Requirements For the Degree of

DOCTOR OF PHILOSOPHY

In the Graduate College

THE UNIVERSITY OF ARIZONA

1 9 7 0 THE UNIVERSITY OF ARIZONA

GRADUATE COLLEGE

I hereby recommend that this dissertation prepared under my direction by _____Darwin Keith Marjaniemi______entitled GEOLOGIC HISTORY OF AN ASH-FLOW SEQUENCE AND ITS SOURCE AREA

IN THE BASIN AND RANGE PROVINCE OF SOUTHEASTERN ARIZONA be accepted as fulfilling the dissertation requirement of the degree of ______Doctor of Philosophy______

Dissertation Director Date

After inspection of the final copy of the dissertation, the following members of the Final Examination Committee concur in its approval and recommend its acceptance:*

O c ^ 7 / 7 O D

This approval and acceptance is contingent on the candidate's adequate performance and defense of this dissertation at the final oral examination. The inclusion of this sheet bound into the library copy of the dissertation is evidence of satisfactory performance at the final examination. STATEMENT BY AUTHOR

This dissertation has been submitted- in partial fulfillment of requirem ents for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowlegment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the m aterial is in the interests of scholarship. In all other instances, however, per­ mission must be obtained from the author.

SIGNED: ACKNOWLEDGEMENTS

This research was carried out while the author was employed in the Laboratory of Isotope Geochemistry at The

University of Arizona. The Laboratory is under the supervision of

Professor Paul E. Damon. Financial support was provided by the

U .S.A .E.C ., under contract AT(11-l)-689, and by the State of

A riz o n a .

The author extends foremost appreciation to Professor

Damon, who suggested the Rhyolite Canyon problem, a problem so challenging and appropriate to the author's interests and abilities.

Professional advice and assistance throughout the research program

is acknowledged.

Members of the Department of Geology assisted on many

occasions. Dr. S. R. Titley advised on several problems and

Dr. Evans B. Mayo's continued interest in the Rhyolite Canyon

problem is acknowledged. Review of the dissertation by the

examination committee is also acknowledged.

The author's colleagues in the Geochemistry Laboratory

assisted in laboratory aspects of the research. Dr. A. W. Laughlin

was influential in the initial development of the author's interest in

iii iv geochemistry, trained him in professional techniques, and assisted in the mass spectrometric analyses. Dennis Coleman and Judith

Percious conducted flame photometric analyses of potassium and also assisted in mass spectrometric analyses. Raymond Eastwood and Robert Scarborough performed the X-ray fluorescence analyses.

Dr. Paul Pushkar attempted to obtain strontium isotopic data on the very low strontium rocks.

The Graduate College, through Dean Bonneville, made available financial support for two weeks of field trips in areas of current ash-flow and caldera research in the Southwest. The author considers that the experience gained from these trips greatly enhanced the efficiency of the research and that this experience was essential to the correct interpretation of the field data. In connection with the field trips, the author is pleased to acknowledge the cooperation and hospitality of volcanologists well-known in ash-flow research:

Robert L. Smith, Ray A. Bailey, Wolfgang E. Elston, Peter W.

Lipman, Thomas Steven, Robert Christiansen, Mike Sheridan, and

Donald W. Peterson. All of these geologists indirectly assisted in the Chiricahua research.

Professor Damon and Robert L. Smith were generous with their time in accompanying the author on a 3-day field trip in the area. Robert Smith shared his interest in the problem throughout the research. Robert A. Zeller provided unpublished information on volcanics in southwestern New Mexico.

Superintendent Ray B. Ringenbach and staff of the

Chiricahua National Monument were helpful at every opportunity.

Residents of southeastern Arizona and southwestern New Mexico were hospitable and helpful, even to the extent of providing informa­ tion on the location of contacts. The Charles Smiths generously made available living facilities in the field area. Brenda M arjaniemi assisted in tasks such as typing and showed admirable interest, understanding and patience. TABLE OF CONTENTS

P a g e

L IST O F IL L U S T R A T I O N S ...... x

L IS T O F T A B L E S ...... x iv

A B S T R A C T ...... x v i

I. I N T R O D U C T I O N ...... 1

N o m e n c la tu r e ...... 3

Location and Physiographic Setting ►tx G eo lo g ic S e ttin g ...... S u b je c t of R e s e a r c h ...... 9 Approach to the Problem ...... 10 P re v io u s W o r k ...... 11 Organization of Research and D issertation ...... 16

II. CHARACTERISTICS OF THE RHYOLITE CANYON F O R M A T I O N ...... 18

III. REGIONAL INVESTIGATIONS ...... 27

IV. TERTIARY GEOLOGY OF THE CHIRICAHUA AND NORTHERN PEDREGOSA MOUNTAINS ...... 31

U n d e rly in g R o c k s ...... 32 M o n u m e n t-C o c h ise H e a d ...... 33 C ave C re e k A r e a ...... 35 H o rs e s h o e C anyon A r e a ...... 41 L o w e r R h y o l i t e s ...... 42 Rhyolite Canyon F o rm atio n ...... 45 U p p er R h y o lite s ...... 46 S hake G u lch A r e a ...... 46 B ru n o C anyon A r e a ...... 49 T u rk e y C re e k C a l d e r a ...... 54 The Distribution of Rhyolite Canyon Formation in the Chiricahua and Northern Pedregosa M o u n ta in s --A S u m m a r y ...... 58

vi v ii

TABLE OF CONTENTS (Continued)

P a g e

V. STRATIGRAPHY IN THE TURKEY CREEK C A L D E R A ...... 61

Rocks Forming the Caldera Walls . 6l R h y o lite C an y o n T u f f s ...... 63 T h i c k n e s s ...... 63 P e t r o g r a p h y ...... 63 Upper and Lower Units: Characteristics an d D i s t r i b u t i o n ...... 65 Characteristics near Contact with the M o n z o n ite ...... 70 Porphyritic R hyolites ...... 74 Moat Rhyolites ...... 75 G e n e ra l C h a r a c te r is tic s and T h i c k n e s s ...... 75 Lower Flows ...... 78 Tuff Breccia and Tuffaceous Sedim ents ...... 81 F in e T u f f s ...... 84 U p p er F l o w s ...... 86 M o n zo n ite P o r p h y r y ...... 90 D om e M o n z o n i t e ...... 90 M o at M o n zo n ite ...... 99 A n d e s i t e s ...... 107 D i k e s ...... 110

VI. COMPARATIVE PETROGRAPHY, CHEMISTRY, AND K-Ar AGE OF ROCKS ASSOCIATED W ITH T H E C A L D E R A ...... 112

H o m o g e n e i t y ...... 112 V e r tic a l V a r i a t i o n s ...... 116

VII. STRUCTURE OF THE CHIRICAHUA MOUNTAINS . . . 119

M a jo r S tr u c tu r a l F e a t u r e s ...... 119 A m o u n t of C a ld e ra S u b s i d e n c e ...... 123

VIII. UNUSUAL FEATURES OF THE TURKEY CREEK CALDERA 127 v iii

TABLE OF CONTENTS (Continued)

P a g e

IX. SUMMARY OF THE TERTIARY GEOLOGIC HISTORY OF THE CHIRICAHUA AND NORTHERN PEDREGOSA MOUNTAINS...... 132

X. DISCUSSION ...... 135

XI. C O N C L U S IO N S ...... 139

APPENDIX I. DETAILS OF REGIONAL RECONNAISSANCE INVESTIGATIONS ...... 141

Central Peloncillo Mts. --Weatherby Canyon (Area 1) . . . 141 S o u th e rn P e lo n c illo M ts ...... 142 South Antelope Pass (Area 2) ...... 143 Northeast of Black Mt. (Area 3 ) ...... 143 Post Office Canyon to Woodchopper's S p rin g (A re a 4 ) ...... 145 Cottonwood Canyon to Clanton Draw (Area 5 ) ...... 147 Perilla M ts. - -College Peaks and P o v e rty F la t (A re a 6 ) ...... 147 S w issh e lm M o u n tain s (A re a 7 ) ...... 148 Sulphur Hills (Area 8 ) ...... 148

APPENDIX II. K-Ar AGE DETERMINATION ...... 149

S am p le P r e p a r a t i o n ...... 149 A n a ly tic a l P r o c e d u r e ...... 150 A n a ly tic a l and S am p le D a t a ...... 150 Discussion of the Analyses of PED -12-62 ...... 154 Discussion of the Discordant Age of DM-4-67 ...... 155

APPENDIX III. PHENOCRYST ANALYSIS ...... 157

Phenocryst Size and Percentage ...... 158 Relative Phenocryst Abundance ...... 158 Q u a rtz S i z e - D i s t r i b u t i o n ...... 161 Discussion of the Origin of Member 5 ...... 162

1 ix

TABLE OF CONTENTS (Continued)

P a g e

APPENDIX IV. X-RAY FLUORESCENCE ANALYSIS .... 167

S am p le P r e p a r a t i o n ...... 167 Analytical Techniques ...... 168 S am p le Id e n tific a tio n and L o c a tio n ...... 169

R E F E R E N C E S ...... 172 LIST OF ILLUSTRATIONS

P la te P a g e

1. Tertiary geologic map and sections of the Chiricahua and Pedregosa Mountains, A r i z o n a ...... In p o c k e t

2. Topographic map of the Chiricahua and Pedregosa Mountains, A rizona ...... In p o c k e t

F ig u r e

1. Location map of southeastern Arizona and southwestern New M exico ...... 5

2. Gemini IV orbital photograph of southeastern Arizona and southwestern New M exico ...... 6

3. Photograph of relief map of Chiricahua Mountains . . . 7

4. Map showing distribution of Tertiary volcanics in southwestern U. S. and northern M exico ...... 8

5. Map of Tertiary volcanics in southeastern Arizona and southwestern New M exico ...... 13

6. Index map of previous work on volcanics in southeastern Arizona and southwestern New Mexico . . 15

7. View in Rhyolite Canyon of members 3, 4, and 6 of the Rhyolite Canyon Formation ...... 18

8. Composite section of Rhyolite Canyon Formation in th e C h iric a h u a N a tio n a l M o n u m e n t ...... 19

9. Composite section of Faraway Ranch Formation in th e M o n u m e n t...... 34

10. View of thick pile of rhyolites composing the Cave C re e k f o r m a t i o n ...... 36

x x i

LIST OF ILLUSTRATIONS (Continued)

Figure Page

11. View of lower portion of the Cave Creek formation . . . 36

12. Section of rhyolites composing the Cave Creek formation near Portal, Arizona ...... 37

13. Flows of Cathedral Rock member overlying interbedded tuffs and flows of Rancho Risco member . . 38

14. Welded ash-flow sheet of the Eagle Cliffs member overlying flows of the Cathedral Rock member .... 39

15. Rugged weathering reddish brown flows of the Painted Canyon m em ber ...... 39

16. Reconnaissance sections in the Horseshoe Canyon a r e a ...... 43

17. Generalized section in the Shake Gulch area ...... 48

18. Rugged weathering reddish brown flows along Sunset fault ...... 51

19. View of lower rhyolites and Rhyolite Canyon tuff in N o rth B ru n o C a n y o n ...... 51

20. Drainage pattern in the caldera...... 56

21. Schematic cross-section of caldera showing s t r u c tu r a l f e a t u r e s ...... 57

22. Photomicrographs of Rhyolite Canyon tuff in the c a ld e ra ...... 66

23. Photomicrographs of welded tuff showing changes as contact is approached ...... 72

24. Brecciation and fluidization of Rhyolite Canyon tuff near an intrusive contact ...... 77

25. View of moat rhyolites in upper Rucker Canyon .... 77 x ii

LIST OF ILLUSTRATIONS (Continued)

Figure Page

26. Generalized section of moat rhyolites in the n o r th e r n p a r t of th e c a l d e r a ...... 79

27. Moat sediments and rhyolite flows in Rucker C a n y o n ...... 82

28. Tuff breccia of the moat sequence below R o ck P e a k ...... 83

29. Bedded tuffaceous rocks exposed in Rattlesnake C a n y o n ...... 85

30. R h y o lite tu ffs of th e m o a t s e q u e n c e ...... 87

31. Uppermost rhyolite flow of the moat sequence o v e rly in g fin e t u f f s ...... 87

32. Hand specimens showing lamination in uppermost rh y o lite flo w s in th e m o a t ...... 88

33. Typical outcrop of monzonite porphyry in the floor of th e c e n tr a l e r o s io n a l c ir q u e ...... 92

34. Relatively fresh hand sample of monzonite porphyry fro m th e d o m e ...... 92

35. P h o to m ic r o g ra p h s of m o n z o n ite fr o m th e do m e ...... 94

36. Mode and norms of monzonite porphyry plotted in tw o c la s s if ic a tio n s y s t e m s ...... 96

37. Fracturing of monzonite near steep contact in th e d o m e ...... 101

38. View of escarpment forming eastern side of caldera. . . 101

39. Hand specimen of monzonite from the m o a t ...... 102

40. Spectacular flow structure in the uppermost part of th e m o a t m o n z o n i t e ...... 102 x iii

LIST OF ILLUSTRATIONS (Continued)

F ig u re

41. Tuffaceous rocks underlying monzonite near the caldera wall in the m o a t ...... 104 i 42. Schematic north-south section through Ida Peak on its south sid e ...... 105

43. Photomicrographs of Long Park an d esite ...... 109

44. Plots of K vs. Rb for caldera sequence ...... 114

45. Plots of Rb vs. Sr for igneous rocks associated with th e c a ld e ra ...... 115

46. Idealized section through Pinery Canyon ...... 124

47. Idealized section through Chiricahua and Sentinel Peaks ...... 125

48. Composite cross-section through the Turkey Creek caldera with hypothetical interpretation .... 129

49. View of overlapping ash flows in South Antelope Pass, Pcloncillo M ountains ...... 145

50. Relative percentages of phenocrysts in Rhyolite Canyon welded tuffs ...... 160

51. Quartz size-distribution in welded zones of the Rhyolite Canyon Formation in the Monument ..... 163

52. Quartz size-distribution in welded zones of the Rhyolite Canyon Formation outside the Monument . . . 164

53. Section through member 5 in the M onum ent ...... 165

54. Quartz size-distribution in zones of Rhyolite Canyon Formation, members 5 and 6 ...... 166 LIST OF TABLES

T a b le P a g e

1. Thickness and Areal Extent of Major Ash-Flow S h e e t s ...... 12

2. Phenocryst Content of Welded Zones of Rhyolite Canyon Tuff in the Chiricahua National Monument . . . 23

3. Chemical Analyses and Norms of Rhyolite Canyon Tuff Compared with Average R hyolite ...... 25

4. Log of Well in Older A lluvium ...... 53

5. Thickness of Rhyolite Canyon Tuff in the Chiricahua and Northern Pedregosa Mountains .... 60

6. Estimated Thickness of Rhyolite Canyon Tuff in th e C a l d e r a ...... 64

7. Partial section of Rhyolite Canyon Tuff below R a ttle s n a k e P e a k ...... 67

8. Estimated Maximum Thickness of Moat Rhyolites E x p o s e d ...... 76

9. Partial Section of Moat Rhyolites in Upper R u c k e r C anyon ...... 80

10. Chemical Analyses on Rhyolite Canyon Formation and M oat R h y o l i t e s ...... 89

11. M ode of M o n zo n ite P o r p h y r y ...... 93

12. Chemical Analyses and Norms of Monzonite P o r p h y ry C o m p a re d w ith A v e ra g e M o n z o n i t e ...... 95

13. K, Rb, and Sr Analyses of Caldera Igneous Units . . . 113

x iv XV

LIST OF TABLES (Continued)

T a b le P a g e

14. K-Ar Ages of Caldera Units ...... 115

15. Estimated Section at South Antelope Pass ...... 144

16. Estimated Section Below Owl P e a k ...... 146

17. K -A r A n a ly tic a l D a t a ...... 151

18. K-Ar Sample Identification and Location ...... 152

19. Petrography of K-Ar Analyzed Samples ...... 153

20. Phenocryst Abundance in Welded Zones of Rhyolite Canyon Form ation ...... 159

21. Stratigraphic Identification and Location of Samples Analyzed for Whole Rock K, Rb, and Sr . . . 170 ABSTRACT

The Tertiary history of the Chiricahua volcanic field of southeastern Arizona is essentially that of rhyolitic ash-flow deposition and concomitant block faulting in the period from 29 to

25 m. y. , as determined by K-Ar analysis. The Rhyolite Canyon ash-flow sheet is the youngest of three sheets, each more than 1000 feet thick. Its distribution is limited mainly to the Chiricahua and northern Pedregosa Mountains with a lesser amount of deposits in the neighboring Swisshelm and Peloncillo Mountains. It is estimated that the original areal extent was of the order of 700 square miles and that the volume of deposits was around 100 cubic m iles.

The source area of the Rhyolite Canyon sheet is identified as a 13-mile diameter caldera, named the Turkey Creek caldera. This is the first major caldera of the Valles type described in the

Mexican Highland and Sonoran D esert sections of the Basin and Range.

It is unique because of its denudation. Erosion to 5000-foot depth locally has exposed thick sections of moat deposits and a fine grained monzonite pluton associated with central doming. Rhyolite Canyon tuff in the caldera, some 3000 feet thick, is domed and intruded by the monzonite. More than 1500 feet of tuff breccia, tuffaceous x v ii sediments, and rhyolite flows are exposed in the moat, along with 3000 feet of monzonite forming annular segments a couple miles wide abutting or overlying rocks forming the caldera wall.

The moat monzonite is sim ilar to that in the dome and was emplaced amidst the period of deposition in the caldera.

Petrographic and trace element analyses indicate a cogenetic relation between the Rhyolite Canyon sequence and the moat rhyolites. The K-Ar age of the Rhyolite Canyon tuff is very close to that of the monzonite.

The ash-flow sheet immediately underlying the Rhyolite

Canyon sheet is also very close in age as indicated by K-Ar analyses.

Block faulting and tilting took place between the two sheets and also following the deposition of the Rhyolite Canyon sheet. There is evidence that the present basin-range structure was not established until after the Rhyolite Canyon sheet had been emplaced. C H A P T E R I

INTRODUCTION

The formulation of concepts relating to the characteristics, origin, and mode of eruption of ash flows (Smith, I960; Ross and

Smith, 1961) has led to their widespread recognition in volcanic provinces throughout the world, and to the identification of large collapse structures or calderas as source areas of major ash-flow sheets. Ash-flow deposits have been identified as forming a major part of volcanic rocks in western North America and some 30 calderas of the Valles type (Smith, Bailey, and Ross, 1961) have been identi­ fied (Smith, personal communication, 1968).

Recent work in the Basin and Range Province has indicated the relative abundance of ash-flow deposits. Several Valles-type calderas have been identified in the Great Basin section but none in the Sonoran Desert and Mexican Highland sections (subdivisions of Fenneman, 1931) which were more severely affected by block faulting. * The latter two sections include southern Arizona,

1. Sheridan (1968) describes two moderate sized calderas in the Goldfield and Superstition Mountains just east of Phoenix, Arizona. Strictly speaking, this is Mexican Highland but it has not been affected by block faulting to the extent that Southern Arizona has.

1 2 southern New Mexico, and portions of California and Texas. The objective of this research was to identify the source area or source caldera of a major ash-flow sheet in this region. The subject: 2 the Rhyolite Canyon Formation in the Chiricahua Mountains of southeastern Arizona.

Why was this problem important? The answer is two-fold.

First, the reason calderas have not been identified in this region must be either their absence due to unusual circumstances associated with eruption of the ash flows, or because of their modification beyond

recognition by tectonic events and erosion. Identification of a source

area would provide information helpful to future ash-flow studies in

this province. If calderas are present their characteristics in this

unique tectonic environment provide basic information on the nature

and origin of calderas. The second reason why this problem is

important is that it should provide information on the interrelation of

ash-flow eruption and caldera formation with the development of this

p ro v in c e .

.2. Chiricahua is an Indian name meaning "mountains of the tu r k e y s " . 3

Nomenclature

Nomenclature relating to ash flows used in this text follows the usage of Smith (I960, pp. 800-801). Terms used repeatedly are defined below after Smith.

ash flow: basic unit of deposition resulting from a single eruption of ash-flow m aterial. This is generally synonymous with the term ignimbrite.

ash-flow sheet: any unspecified sheetlike unit or group of units of ash-flow origin.

welded tuff: a rock or rock body in which vitric particles have some degree of cohesion by reason of having been hot and viscous at the time of their emplace­ ment. In this text the term is used synonymous with welded ash-flow tuff.

cooling unit: a single or multiple ash-flow deposit that can be shown to have undergone continuous cooling.

ash-flow field: deposits of pyroclastic rocks, con­ sisting preponderantly of ash flows, which are related to some specific unit of area.

The following term s relating to calderas are also defined:

caldera: "large volcanic depressions, more or less circular or cirque-like in form, the diameters of which are many times greater than those of the included vent or vents, no m atter what the steepness of the walls or form of the floor" (Williams, 1941, p.251).

resurgent caldera: a caldera whose central portion, at least, has the structural form of a dome as a result of post-subsidence uplift of the floor. 4

Location and Physiographic Setting

The area of study is located in southeastern Arizona and

southwestern New Mexico (Figure 1). In this part of the Basin and

Range, north-trending ranges, 10 to 15 miles wide, rise up to 6000 feet in relief above intervening valleys or basins that are 5 to 10

miles wide. The general physiography of the region can be seen in the

Gemini photograph in Figure 2. More detailed physiography of the

Chiricahua Mountains, the area of prim ary interest in this research,

can be seen in the relief map in Figure 3.

The physiography of this area is advantageous to the geologist

in that (1) it allows ready access to the ranges through the inter­ vening valleys, (2) thick sections are exposed by canyons cutting

transverse to the trend of the ranges, and (3) elevation differences

perm it year-around field work, i. e. , low elevations in the winter

and high elevations in the summer.

Geologic Setting

The Chiricahua and Swisshelm Mountains (Figure 1) are

the westernmost ranges in this part of the Basin and Range Pro­

vince that are composed largely of Tertiary volcanics (see Figure4).

Each of these ranges is virtually isolated and may be considered a

volcanic field. They form part of a gigantic volcanic province which

includes the Sierra Mad re Occidental of Mexico and the Mogollon 5

Lords burg

- 32° Sulphur ,Moo .} y X Hills

^XBisbce

Douglas’ MEXICO

20 MILES

FIGURE 1. - -Location map of southeastern Arizona and south­ western New Mexico. Dashed line outlines area of study. Solid line indicates area of map (Plate 1). 6

FIGURE 2. - -Gemini IV orbital photograph of southeastern Arizona and southwestern New Mexico. From altitude of about 100 miles. Chiricahua Mountains (A) and southern Peloncillo Mountains (B). Dots outline Turkey Creek caldera. Also showing the San Bernar­ dino basalt field (C). National Aeronautics and Space Administra­ tion photograph. 7

FIGURE 3. - - Photograph of relief map of Chir icahua Mountains. Dots outline Turkey Creek caldera. U. S. Army Map Service map. FIGURE 4. - - Map showing distribution of Tertiary vol­ canoes in southwestern U. S. and northern Mexico. Arrow points to Chiricahua volcanic field. From Geologic Map of North A m erica, 1965. 9

Plateau. This province is characterized by thick sections of silicic ash-flow deposits of Miocene-Oligocene (m id-Tertiary) age

(King, 1939, p. 1679; W isser, 1966; Elston, Coney, and Rhodes,

1968). The area of study essentially lies in the intersection of the

Sierra Madre Occidental ash-flow province and the Basin and Range tectonic province. Many ranges in this region are gigantic uplifted fault blocks, displaced as much as a mile or more from adjacent valley blocks along north-trending faults.

Subject of Research

As already stated, the objective of this research was to identify the source area or source caldera of the Rhyolite Canyon

Formation. 3 The basis for thinking that this was feasible was the following (Damon, personal communication, 1966):

1. Petrographic worked on the Rhyolite Canyon Formation had been done by Enlows (1955). Enlows described a petrographically homogeneous sequence of ash flows 1700 feet thick. Although this work was confined to only about 15 square miles it was postulated

3. Constitution of the Rhyolite Canyon Formation will be described in the next chapter. It is noted here that the uppermost member of the formation is a lava flow and that since this research is concerned prim arily with the ash flows, throughout the text the term Rhyolite Canyon tuff, Rhyolite Canyon Formation, or Rhyolite Canyon ash flows will be used as meaning that major portion of the formation which consists of ash-flow deposits. 10 that such a thick sequence of ash flows could be expected to have a volume in the range of deposits related to known calderas (Smith,

I960, p. 819), i. e. , more than a few cubid miles.

2. A potassium-argon age of 16.2 (m. y. ) million years obtained on the Rhyolite Canyon ash-flow sheet (Damon et al, 1962, p. 25) indicated that it was one of the youngest in the region, appreciably younger than the m id-Tertiary peak of 27. 5 m. y. (1

7. 5 m. y .) (Damon and Mauger, 1966, p. 100). On this basis, it was thought that the Rhyolite Canyon sheet would have been spared the main thrust of basin and range tectonic activity so that it would be a good subject for determining the source caldera. With this age it would also be easily distinguished by K-Ar dating (expected mean standard deviation around 0. 7 m .y. ) from the ubiquitous volcanics around the m id-Tertiary peak. Although the 16. 2 jd . 6 m .y. age of the

Rhyolite Canyon Formation was shown by the author to be in error due to impurities in the m ineral separate (discussed in Appendix II), and the true age found to be 24. 9 +0. 6 m .y. , the relative youth of this ash-flow sheet was verified and this relation proved to be very beneficial in solving the problem.

Approach to the Problem

Previous workers in southeastern Arizona and southwestern

New Mexico attempted to identify local sources for the ash flows 11 within areas that they mapped, areas on the order of tens of square m iles. With the knowledge of the nature and mode of eruption of ash flows that was now available (Smith, I960; Ross and Smith, 1961), the author felt that a regional approach was warranted. It is known, for example, that ash flows may be continuous for more than 50 miles

(Smith, I960, p. 814) and that they cover as much as 10, 000 square miles over favorable topography (Cook; 1965).

Before undertaking this problem, an attempt was made to estimate the magnitude of the area covered by the Rhyolite Canyon sheet. From published data it was noted that ash-flow sheets with

1/3 to 1/10 the thickness of the Rhyolite Canyon sheet (1700 feet thick in the Chiricahua National Monument) covered thousands of square miles (Table 1). Pre-eruption topography in southeastern

Arizona and southwestern New Mexico was surely not very favorable because of contemporaneous block faulting. Nevertheless it was considered likely that Rhyolite Canyon ash flows covered on the order of several thousand square miles.

Previous Work

Enlows (1955) described the petrography of the Rhyolite

Canyon Formation and mapped it in an area of less than 15 square miles in the Chiricahua National Monument (Figure 1). His work constituted the state of knowledge of the Rhyolite Canyon Form ation 12

T A B L E 1 -Thickness and Areal Extent of Major Ash-Flow Sheets

A sh -flo w A ve. D e p o sitio n A re a l R e fe re n c e s h e e t th ic k . to p o g ra p h y e x te n t (ft.) (m i.2)

Whakamaru Ignimbrite, 175 basin; 900 ft. 1000 Ewart, 1965 Taupo Volcanic Zone, r e lie f New Zealand

Topopah Spring Member, 300 broad trough, 700 L ip m a n , Paintbrush Tuff, several hun­ Christiansen, Southern Nevada dred ft. local and O'Connor, r e lie f 1966

Yellowstone Tuff, 500 canyons,600 1600 B oyd, 1961 W yom ing ft. r e lie f

Eastern Nevada 150 b ro a d 4 0 0 0 - C ook, 1965 v a lle y s 6000

prior to the author's work. Petrographic work by Enlows provided

essential groundwork for the author's work.

The extent of mapping of Tertiary volcanics in south­

eastern Arizona and southwestern New Mexico is essentially that

found on the Geologic Map of Cochise County (Arizona Bureau of

Mines, 1959) and the Prelim inary Geologic Map of Southwestern

New Mexico (Dane and Bachman, 1961), reproduced in Figure 5.

The author would like to underscore the reconnaissance nature of

this map, noting that appreciable areas are neither volcanic nor

rhyolitic as indicated. 13

Chin O Sulphur Hills

10 MILES

MEXICO

EXPLANATION

ARIZONA NEW MEXICO

(After Arizona Bureau of Mines, (After Dane and Bachman, 1961, 1959 with m inor modification) with minor generalization) □ I Tvu I Rhyolites Rhyolite Flows Includes m inor quartz latite and Includes welded rhyolite tuffs, rhyo­ andesite litic pyroclastics, white tuffaceous sandstone, and subordinate trachytic latite and felsite □ D Tertiary volcanics undiff. ED Andesite and Basalt Flows

Includes subordinate amounts of latite, quartz latite, dark red rhyolite, and rhyolite pyroclastics

FIGURE 5. - - Map of Tertiary volcanics in southeastern Arizona and southwestern New Mexico. Areas of reconnaissance investigations referred to in Appendix I are indicated by num bers. 14

An index map of previous work on volcanics in this area is included as Figure 6. Enlows (1955) described the Rhyolite

Canyon Formation, and Fernandez and Enlows (1966) described the underlying Faraway Ranch Formation in the Chiricahua National

Monument. Sabins (1957) mapped these formations over a slightly larger area to the north and described some additional units.

Raydon (1952) subdivided the Tertiary rhyolites into four members and mapped them over about 10 square miles in the Portal area. He included generalized petrographic descriptions. Epis

(1956) mapped the Tertiary rhyolites as a unit in the Pedregosa and

Chiricahua Mountains. He included a general petrographic descrip­ tion. Cooper's reconnaissance map (Cooper, 1959) incorporated data of Raydon and Epis. His map was of great value in.this research, providing a small scale base map of Tertiary rhyolites.

Gilluly (1956) mapped the Tertiary volcanics in the hills around

Pearce, Arizona, west of the Chiricahua Mountains, and described the section in some detail.

In New Mexico, Gillerman (1958) mapped and described the

Weatherby Canyon ignimbtite in an area of less than 10 square miles

in the central Peloncillo range. Wrucke and Bromfield (19 61) mapped

several welded tuff units in reconnaissance fashion in the southern 15

I______I

I______

MEXICO

109*30' 109*00'

0 10 20 30 Miles 1 ______|______i______|

1. Enlows, 1955, report and map; Fernandez and Enlows, 1966, map and report. 2. Sabins, 1957, report and map. 3. Cooper, 1959, reconnaissance map. 4. Epis, 1956, report and map. 5. Raydon, 1952, report and map. 6 . Gilluly, 1956, report and map. 7. Gillerman, 1958, report and map. 8 . Wrucke and Bromfield, 1961, reconnaissance map. 9. Dane and Bachman, 1961, regional map.

F IG U R E 6 . --Index map of previous work on volcanics in south­ eastern Arizona and southwestern New Mexico 16

Peloncillo Mountains. This map was of some use although appreciable areas of lava flows were mapped as welded tuffs.

Organization of Research and Dissertation

The organization of the dissertation generally follows that of the research, which included the following phases:

1. A study of the Rhyolite Canyon Formation and the establishment of correlation criteria. At the start of this research, the entire volcanic section in the Monument was studied and sampled.

K-Ar ages were obtained from near the base and top of the Rhyolite

Canyon Formation. From this work, criteria were selected which could be used for correlation of the Rhyolite Canyon tuff. This phase involved extensive laboratory work (K-Ar and chemical) which continued throughout the period of research as individual correlation problems were encountered. Chapter II includes a description of the distinguishing characteristics of the Rhyolite Canyon Formation.

2. A determination of the regional extent of Rhyolite

Canyon tuffs. In this phase, key volcanic sections located throughout the 3000-square mile area of study (Figure 1) were examined.

Petrographic and K-Ar analyses were made. About 15 days were involved in the field work.

3. Mapping and field studies of rhyolites in the Chiricahua and northern Pedregosa Mountains. This phase involved field 17 mapping and studies over an outcrop area of about 300 square m iles. Petrographic and K-Ar analyses were also made. This and the subsequent phase involved 65 days of field work, divided about evenly.

4. A study of the caldera. This phase involved mapping and studies in the 100 square mile caldera. Petrographic, K-Ar, and chemical analyses were also conducted.

From field trips in areas of current volcanological research

in the Southwest, the author gained a basic knowledge of volcanic geology, ash flows, and calderas essential to the conduct of this

r e s e a r c h .

In order that this report may be of optimum benefit to future workers in the area, a significant amount of descriptive

m aterial and analytical data is included which may not be of interest

to most readers. Some of this data has been relegated to the

appendices.

The reader is reminded that, by the nature of the objectives,

this study cannot be considered an area study, not even of Tertiary volcanics within a specific area. Evidence relating to the history

and source area of the Rhyolite Canyon Formation has been sought,

at the expense of completeness or uniformity of geologic knowledge

in any one area. CHAPTER II

CHARACTERISTICS OF THE RHYOLITE CANYON FORMATION

The Rhyolite Canyon Formation was named by Enlows (1955) after Rhyolite Canyon in the Chiricahua National Monument, along which canyon the welded tuffs are exposed in spectacular columns

(Figure 7). Enlows subdivided the formation into 9 members (Figure 8).

Eight of the members are porphyritic rhyolite ash-flow tuffs. A rhyodacite flow caps the section.

FIGURE 7. - -View in Rhyolite Canyon of m em b ers 3, 4, and 6 of the Rhyolite Canyon For­ mation. Showing columnar jointing and horizontal partings characteristic of members 4 and 6.

18 19

Thick. Member, Description (ft. ) rock type______

Black to medium gray; 220 9. rhyodacite flow v e s ic u la r Soft; light gray, grading into / 50 8. rhy. welded tuff brittle, brown rock downward \ 1 0 7. rhyolite tuff Fairlv coherent, light grav

Rather coherent light gray tuff, grading down into poorly 880 6 . rhy. welded tuff welded light gray tuff , which in turn grades into a coherent, brittle, light brownish gray ro c k

5. rhy. welded tuff Glassy, light brownish gray Soft light gray top with strongly welded grayish red seams, 270 4. rhy. welded tuff grading down into coherent brittle pinkish gray rock

Firm ly welded, dusky red,with 190 3. rhy. welded tuff many prominent inclusions

Soft light gray top grading into 270 2. rhy. welded tuff a brittle grayish red rock v e r y 1. rhy. welded tuff fi& \ye'wf6fekdgray and y . *• A • •• h * * V; Faraway Ranch Formation, rhyolite sillar / • f .

F IG U R E 8. --Composite section of Rhyolite Canyon Formation in the Chiricahua National Monument. After Enlows, 1955. 20

The formation consists of at least 3 separate cooling units, as indicated by two unconformities in the section. It is not known whether cooling breaks are present in the bulk of the formation, which includes members 2 th ro u g h 6.

In this study, the author worked mainly with the major wel­ ded units of the formation, members 2, 3, 4, 6 , an d 8 . Discussions that follow reflect this. Member 5 is probably a "subflow" (Smith,

I960, p. 811) of member 6 (see discussion in Appendix III).

The Rhyolite Canyon tuffs are described here prim arily in term s of characteristics which allow one to distinguish them from other welded tuffs in the area, and characteristics which distinguish one member of the formation from another.

1. Joints, Fractures and Weathering Characteristics.

Columnar jointing and horizontal fractures commonly assumed by welded tuffs characterize members 4 and 6 in the Monument (Figure

7). The spectacular rock formations for which the Monument is known are formed in these members. Members 2 and 3 are massive, slope-formers. Member 3 shows some columnar jointing in steep canyons (Figure 7). The welded portion of member 8 has the appear­ ance of a cliff-forming rock though it is only 40 feet thick in the

Monument. It must have been hot when emplaced to be welded while being relatively thin. 21

Horizontal fractures or partings resulting from compaction of pumice fragments are always present to some degree in the tuffs, being most prominent in the upper portions of members 4 and 6 (F ig ­ ure 7). Partings may be somewhat irregular but are generally parallel. These are helpful in distinguishing welded tuffs from flows at a distance. In addition, cliffs in flows are generally less planar than welded tuffs, exhibiting some degree of concavity from w eather­ ing along flow layers.

Fractured and weathered blocks of Rhyolite Canyon welded tuff are distinctly more cubic than the granitic-weathering rhyolites and quartz latites of the southern Peloncillo Mountains.

2. Color. When exceeding about 200 feet in thickness,

Rhyolite Canyon tuffs are typically light brownish gray! to pale brown in basal portions, grading into light gray or light browish gray toward the top. This color change is the normal consequence of increased crystallization and devitrification upward in the section.

An exception to this is member 3 which is dusky red throughout.

Color can be extremely variable and misleading, but, in that it is an expression of such things as composition, crystallization, alteration, etc. , it can be used along with other characteristics in

1. Color designations are in accordance with the National Research Council Rock-Color Chart. 22 quick recognition of the formation. Where the tuffs are hydrother- mally altered or crystallized, such as in the caldera, colors are anoma­ lous, ranging from red-browns resulting from oxidation, to light and dark gray due to crystallization of the groundmass.

3. Inclusions. Eutaxitic texture is almost universally apparent in the Rhyolite Canyon tuffs, except where they are crystal­ lized near an intrusive contact. These lenticles may amount to 25 percent of the rock.

Lithic inclusions rarely amount to more than a few percent outside the caldera, except in member 3 which contains up to 20 percent. This characteristic is very useful in distinguishing Rhyolite

Canyon tuffs from older lithic tuffs common to the northern Pedregosa and southern Peloncillo Mountains. Lower portions of Rhyolite

Canyon in the caldera are anomalous in containing up to 30 percent lithic fragments.

4. Phenocrysts. Phenocryst content in the Chiricahua

National Mounument ranges from 5 to 35 percent but is typically 15 to 25 percent (Table 2). Sanidine and quartz, up to 4 mm in size, comprise most of the phenocrysts, Sanidine exceeds quartz in volume.

Magnetite is usually present and may amount to 5 percent. Biotite is found in member 2 but is less than 1 mm in size and less than 1 p e r ­ cent by volume. 23

TABLE 2. --Phenocryst Content of Welded Zones of Rhyolite Canyon Tuff in the Chiricahua National Monument

M e m b e r Minerals Volume M ax. s iz e % (m m )

8 quartz and sanidine 2 0 -3 5 4. 0 magnetite 4-5 2 . 0

6 quartz and sanidine 2 0 -3 0 3. 0 magnetite 2-3 0 . 5

4 quartz and sanidine 10-16 3. 0 m a g n e tite 2 -3 0. 5

3 quartz and sanidine 15-23 2 .5 m a g n e tite 1-2 1.0

2 quartz and sanidine 5 -1 5 2 . 0 m a g n e tite 1-2 0. 5 b io tite 1 . i . o

In thin section, sanidine occurs as large euhedral to

subhedral crystals and angular fragments over a range of sizes.

Quartz is commonly euhedral to rounded and embayed, more abun­

dant in larger fragments since it is less easily fractured than

sanidine. Magnetite is rectangular.

Total phenocryst content, relative abundance, and size are

valid criteria for identification of Rhyolite Canyon tuff. In practice,

field correlations on the basis of megascopic characteristics were

verified by thin section examination or determination of the relative 24 percentage of phenocrysts present in stained slabs. ^ This technique has been used by Mackin (I960), Cook (1965) and W illiams (I960) to correlate ash flows over considerable areas in the Great Basin.

5. Age and Stratigraphic Position. K-Ar ages of 25. 0 +0. 8 and 24. 9 +0. 7 m. y. were obtained on sanidines from members 2 and

8 respectively. 3 These ages fall well within \< f(7 . 5 m. y. ) of the mid-

Tertiary peak (27. 5 m. y. ) in this part of the Basin and Range (Damon and Mauger, 1966, p. 100).

The Rhyolite Canyon ash-flow sheet is the youngest sheet in the area. This relation is beneficial to the problem of correlation and identification.

6 . Chemistry. Chemical analyses on member 6 , reported by Enlows, are reproduced in Table 3 along with Nockolds 1 a v e r a g e calc-alkali rhyolite. Member 6 is notably richer in Si02 and poorer

in CaO than the average rhyolite. The unusually low Ca content of m e m b e r 6 w arrants its classification as a calcic-poor rhyolite.

It is noted that the composition of member 6 compares more favorably with Nockolds 1 average alkali rhyolite. There is no evidence

2. See Appendix III for staining procedure.

3. K-Ar analytical procedure and data, and sample informa­ tion may be found in Appendix II, A discussion of a previously ob­ tained discordant age on the Rhyolite Canyon Formation is also in this appendix. 25

TABLE 3. - -Chemical Analyses and Norms of Rhyolite Canyon Tuff Compared with Average Rhyolite

Rhyolite Canyon Fm. Average calc- m e m b e r 6 alkali rhyolite (1) (2)

S i c 2 76. 38 7 3 .6 6 A1203 1 2 .4 7 1 3 .4 5 F e 20 3 1 .5 6 1 .2 5 F eO T r . 0. 75 M gO 0. 15 0 .3 2 C aO 0. 15 1. 13 N agO 3 .0 4 2 . 99 k 2 o 4 .8 5 5. 35 h 2 o 1. 11 0. 78 T iC 2 0. 12 0 . 22 P 2O5 0. 01 0. 07 M nO 0 .0 5 0. 03 9 9 .8 6 100.00

O r 28.91 3 1 .7 Ab 2 5 .1 5 25. 1 A n 0. 83 5. 0 Mg 1 .9 11 0. 5 H m 1.60 Ap 0 . 2 C 2. 04 0. 9 Hy 0 .4 0 M gSiO s 0. 8 F e S iO s Q 3 9 .7 8 33. 2 9 8 .7 1 99. 3

(1) E n lo w s 1 sample 1, 1955, p. 1233

(2) Nockolds 1 average of 22 analyses, 1954, p. 1012 26 of alkalic affinity such as the presence of feldspathoids, however, nor is there an excess of Na+K beyond that needed to form the feldspars which would result in the formation of feldspathoids.

Results of K, Rb and Sr analyses on the entire ash-flow sequence will be considered in Chapter VI. Suffice it to say for now that K content ranges from 4. 04 to 4. 35 percent, Rb from

301 to 417 ppm, and Sr from 7 to 19 ppm. The unusually low Sr content and high Rb/Sr ratio are notably characteristic of this s h e e t.

In summary, characteristics which are most useful in rapidly identifying Rhyolite Canyon tuffs include: (1) color, usually light gray to light brownish gray, (2) inclusions, generally less than a few percent, (3) phenocrysts, usually between 15 and 25 percent, with quartz and sanidine in about equal amounts, and no biotite. If biotite is present, it will likely be less than 1 mm in

size, less than 1 percent by volume, and accompanied by 5 to 15

percent quartz and sanidine phenocrysts not exceeding about 2 m m

in size (this is the case in member 2). CHAPTER III

REGIONAL INVESTIGATIONS

Initial reconnaissance in the Chiricahua and northern

Pedregosa Mountains revealed thick sections of Rhyolite Canyon tuffs exposed in widely separated areas. In Bruno Canyon, 20 miles south of the Monument in the northern Pedregosa Mountains, a minimum of 1500 feet is exposed. In Shake Gulch, 7 miles directly east of Bruno Canyon, at least 500 feet is exposed.

With such great thicknesses exposed in the Chiricahua

Mountains, it was considered likely that Rhyolite Canyon tuffs would be found in the neighboring ranges, especially in the southern

Peloncillo Mountains where overlapping ash-flow sheets, reaching thousands of feet in total thickness, can be seen from the San Simon

V alley .

Results of reconnaissance investigations of key sections in ranges neighboring the Chiricahua range are summarized here. . Details may be found in Appendix I. From these investigations, the following conclusion was arrived at. Outcrops of Rhyolite Canyon tuffs are confined to the Chiricahua and Pedregosa Mountains with the following exceptions:

27 2 8

1. Possible correlation with welded tuffs in the W eather- by Canyon area (location^ 1). A K-Ar age of 26,3 +0. 8 m. y. from

the base of the section and petrography does not preclude the

possibility of correlation of at least a part of the 2000-foot thick

section with Rhyolite Canyon tuff. Overall lithology is not entirely

in agreement with such a correlation. Some units appear correlative

with pre-Rhyolite Canyon tuffs.

2. A thin outcrop of Rhyolite Canyon tuff at South Antelope

Pass^ (location 2). Lithology and phenocryst content are sim ilar to

that of a lower member in the Chiricahua National Monument.

3. Several hundred feet of Rhyolite Canyon tuff in the

Swisshelm Mountains (location 7). Lithology and phenocryst content

of this rock are sim ilar to that of member 8 in the Chiricahua

National Monument.

The occurrence of Rhyolite Canyon tuff at South Antelope

Pass is most significant because it establishes a time-horizon for

both the volcanic and structural history of the Peloncillo and neighbor­

ing ranges. Regarding the volcanic history, the outcrop of Rhyolite

1. Locations are indicated in Figure 5.

2. Topographic maps in this area use the name Antelope Pass for two different locations. In this text, they are differentiated by adding the designation North or South; South Antelope Pass being the one directly east of Rodeo, New Mexico. 29

Canyon was found at South Antelope Pass in an area mapped Tvu

(Figure 5; location 2), immediately above what Zeller identified as

Gillespie Tuff (Zeller, personal communication, 1967). This would date the lower unit (Tvl) and the Gillespie Tuff as pre-Rhyolite

Canyon. The Gillespie Tuff is an important stratigraphic unit in southwestern New Mexico, where it has been widely mapped by

Zeller (see Zeller and Alper, 1965) and used as a stratigraphic m arker between the upper and lower volcanics (Tvu and Tvl, respecr tively). Most of the southern Peloncillo Mountains is mapped as Tvl

(Plate 1) and this would mean it is made up of pre-Rhyolite Canyon 3 rocks. It is possible that a thick sequence of Rhyolite Canyon tuffs capped the Southern Peloncillo Mountains at one time but were quickly removed from the uplifted range block, unprotected by younger volcanics or sediments, and more susceptible to erosion, so far from the source (being less densely welded).

Concerning the structural history, the occurrence of

Rhyolite Canyon tuff at South Antelope Pass dates the uplift of the

Peloncillo range as post-Rhyolite Canyon, since otherwise the ash

3. This is also supported by lithologic similarities of rocks at South Antelope Pass and Cottonwood Canyon (locations 2 and 5, respectively) with rocks underlying Rhyolite Canyon welded tuffs in the Horseshoe Canyon area (especially T19S, R31E), only 10 miles to the west. flows would have filled the intervening valley to more than 5000 feet before being deposited on the crest of the Peloncillo range. CHAPTER IV

TERTIARY GEOLOGY OF THE CHIRICAHUA

AND NORTHERN PEDREGOSA MOUNTAINS

A description of the Tertiary geology of the Chiricahua and

Pedregosa Mountains is made difficult by: ( 1) size of the area; area covered by the geologic map (Plate 1) includes portions of six 15- minute quadrangles with a total enclosed outcrop area of about 550

square miles, (2) ubiquitous block faulting, quasicontemporaneous with extrusion of units mapped, (3) local nature of many of the units, a consequence of both the limited extent of units such as silicic lavas and erosion of uplifted fault blocks, (4) the large number of units;

commonly, more than a dozen units may be found, and (5) diagenetic

and hydrothermal alteration and weathering. In view of this com­

plexity, correlation of units over the total area mapped was not

possible for the most part with the exception of the Rhyolite Canyon

tuff. Consequently, the Tertiary geology is described by the follow­

ing areas or structural-stratigraphic subdivisions: (1) Monument-

Cochise Head, (2) Cave Creek, (3) Horseshoe Canyon, (4) Shake

Gulch, (5) Bruno Canyon, and (6) Turkey Creek caldera. These

are indicated on an index map at the bottom of Plate 1. T h e s e

31 32 subdivisions are essentially massive fault-bounded blocks, whose isolation has been enhanced by erosion (see Figure 3).

The central structure in the area is a 13-mile diameter caldera. South and east of this are three north-south elongated fault blocks, 4 to 6 miles across. These are the Cave Creek and

Horseshoe Canyon areas, combined; Shake Gulch area; and Bruno

Canyon area. They are bounded by majoi* ndrth-south vertical faults the Horseshoe, Winn, Sunset, and Cholla faults. The Cochise Head area is isolated from the Monument by the prominent northwest­ trending Apache Pass fault zone (Sabins, 1957).

Underlying Rocks

Pre-Tertiary rocks in the Chiricahua and northern

Pedregosa Mountains consist prim arily of andesitic lavas, breccias, and associated sediments. They are considered Cretaceous in age by Cooper (1959) on the basis of fossil evidence. These rocks are

found exposed only on the eastern slopes of the westward dipping

fault blocks.

In the Horseshoe Canyon area, the Tertiary rhyolites are

apparently in depositional contact with a small stock which intrudes

the andesites. 33

Monument - Cochise Head

The Tertiary geology of this area has been summarized by Marjaniemi (1968). Tertiary rocks were divided into two form a­ tions by Enlows (1955). The lower is the Faraway Ranch Formation, and the upper, the Rhyolite Canyon Formation. The form er is a heterogeneous sequence of volcanics and volcanic -derived sediments ranging from rhyolite to andesite (Figure 9). Detailed petrographic work on the formation was done by Fernandez and Enlows (1966).

Total thickness is 1375 feet. The author obtained K-Ar ages of

28. 9+1.9 m. y. and 27. 7 +^0. 7 m. y. on members 2 and 7, respec­ tively. * This would place the bulk of the formation in Oligocene time, just around the mid^-Tertiary peak (Damon and Mauger, 1966, p. 100).

Rhyolite Canyon ash flows, totalling 1927 feet in thickness (Enlows,

1955), unconformably overlie the Faraway Ranch Formation.

Rhyolite Canyon tuff is not known to be present in the

Cochise Head area. Cochise Head is formed in a rhyodacite flow

(Sabins, 1957) which is very likely Faraway Ranch member 7.

Below this, on the eastern slopes of Cochise Head and M averick

Peak, at least four ash flows of rhyolite composition^ may be seen.

1. Sample information and analytical data may be found in Appendix II.

2. Classification of volcanics throughout this text is on the basis of phenocryst composition assessed by hand-lens examination unless data presented indicates otherwise. Thick.(ft. ) Member Description

Rhyolite Canyon Fm. , rhyolite welded tuff 55 9. rhy. sillar Very light gray, porous Moderate reddish, brown, porous, 8. rhy. sillar poorly consolidated; containing V 5 many inclusions Pinkish gray to pale reddish brown, hard, marked by promi­ 500 7. rhyodacite nent flow structure; grades from flow dense, glassy dark gray base to grayish pink scoriaceous top

Coarse volcanic breccia and 200 6 . v o lc a n ic conglomerate overying volcanic fanglomerate sandstones and breccia

Medium gray to dusky red andesite 330 5. basaltic and. clasts in a grayish orange matrix + v <• b r e c c ia A -h A + A + / 110 4. basaltic and. 4- + t flow & breccia Dark gray, porphyritic / 65 3, rhy, tuff Grayish pink, porphyritic / 40 2. air fall tuff Pinkish erav, fine-grained 1. lithic eray- Grayish pink, coarse grained y 50 w ack e sa n d s to n e r J A n d e site

FIGURE 9 • - -Composite section of Faraway Ranch Formation in the Monument. After Fernandez and Enlows, 1966. Member 4 has been reclassified as a basaltic andesite on the basis of high potassium content as indicated by beta activity. 35

Phenocrysts are generally less than 15 percent, biotite is present up to 2 percent. Total thickness of these tuffs is of the order of

2, 000 fe e t.

Cave Creek Area

The Cave Creek formation was named by Raydon (1952) for

the thick sequence of rhyolites which forms spectacular cliffs and formations along Cave Creek, near Portal,Arizona (Figures 10 and

11). Raydon subdivided the sequence into four members which he

called welded tuffs. The author examined this sequence and found

that two of the members consisted of flows and tuffs rather than

welded tuffs (Figure 12).

According to Raydon, relative phenocryst abundance in the

rhyolites is quite uniform. About 50 percent of the phenocrysts are

sanidine, 40 percent quartz, and the remainder biotite or pyroxene.

Iron oxide grains are present throughout. Phenocrysts are 1 to 2 mm

on the average, but may be as large as 5 mm in the Rancho Risco

member. The petrologic homogeneity of the sequence is suggestive

of consanguinity.

Raydon mapped the formation over an area of about 10

square m iles. In reconnaissance studies of this sequence the author

encountered it over a much larger area to the south and noted the

following additional characteristics: 36

FIGURE 10. --View of thick pile of rhyolites composing the Cave Creek formation. Rugged cliffs on the horizon are youngest rhyolite flows, brilliantly colored pink to dark red. Slopes in foreground are Cretaceous sedimentary rocks. Total relief: 3000 feet. Looking east from caldera . Cave Creek is in the center.

FIGURE 11. - - View of lower part of the Cave Creek formation. Bedded units (B) form the base of the Tertiary section and overlie Cretaceous andesites (A). Looking west from Portal, Arizona. 37

Rock Description Member type thick.(ft. )

Forming bright red or Vermil­ lion, rugged cliffs; very hard due Painted to silicification; fine-grained. Canyon flows Slightly porous tuff at top. Grayish white, laminated base. 900

Cliff-forming, vertically jointed; hard, light brownish gray. Top Eagle welded is light gray pumiceous tuff, Cliffs tuffs 850- 900

Cavernous weathering, hard; flow structure up to hundreds of Cathedral feet high; medium grained, light Rock porphy- gray to light pinkish gray, speck­ r itic led by num erous tiny spherulites . 1250 - flows Light gray pumiceous tuff at top. 1300 Spherulites and pitchstone at base. pumiceous Fairly hard X tuff flows & welded Finely to coarsely porphyritic; Rancho tuffs (?) light gray to reddish brown. ^ tu ff or Risco ^ flow 1400 Includes tuffs that are water-laid; fine to coarse grained; light gray tuffs to grayish yellow and light green­ ish gray. Blacktail Formation- andesitic and dacitic lavas, tuffs, breccia, and sediments

FIGURE 12. - -Section of rhyolites composing the Cave Creek Formation near Portal, Arizona. Modified from Raydon, 1952. 38

1. Flow -banded rhyolites are common in the Rancho Risco and Cathedral Rock members (Figures 13 and 14). In the latter, folds may be several hundred feet high.

2. The Painted Canyon member is distinguished by its striking coloring and weathering characteristics (Figure 15). Exten­ sive oxidation and silicification give this rock a varicolored appear­ ance, ranging from light gray tones to moderate reddish brown in fresh sample. It fractures in small blocks and sheets and has some horizontal fractures, probably following horizontal flow layers.

FIGURE 13. --Flows of Cathedral Rock member (B) overlying interbedded tuffs and flows of Rancho Risco member (A). Also showing the landmark Cathedral Rock (C) which has the appearance of a volcanic neck. View looking west-southwest from Portal, Arizona. 39

FIGURE 14. - -Welded ash-flow sheet of the Eagle Cliffs member (B) overlying flows of the Cathedral Rock member (A). View looking north from junction of Cave Creek and South Fork.

FIGURE 15. - -Rugged weathering reddish brown flows of the Painted Canyon member. North wall of South Fork. 40

3. Painted Canyon member is distributed along a zone which borders South Fork for about one mile to the northwest and about two miles to the southeast. It is found beyond the end of South Fork, within a few hundred feet of contact with the Cretaceous while under­ lying members are absent or very thin. The lower members appear to have been deposited against a moderately sloping surface or perhaps on the edge of a basin.

In the author's investigations, no exposures of Rhyolite

Canyon tuff were found in the Cave Creek area. In order to establish the age of the Cave Creek formation relative to the Rhyolite Canyon

Formation, for purposes of mapping, K-Ar ages were obtained.

An age of 21.4 +0. 6 m. y. (DM-4-67) was obtained on a sample from very near the base of the formation in the Rancho Risco member. An

age of 25. 7 +^0. 8 m. y. (DM-6-67) was obtained on a sample from

the overlying Eagle Cliffs member. The reasons for the discordance

of the form er age are discussed in Appendix II. The latter age is

consistent with the following field evidence for a pre-Rhyolite Canyon

age for the formation:

1. Rhyolite Canyon tuff is absent in the Tertiary section

exposed along Cave Creek and South Fork. 41

2. Rhyolite Canyon tuff outcrops just west of Horseshoe

Pass. The outcrop is located only a few miles southeast of South

Fork, at an elevation above the Painted Canyon member. Barring an intervening fault this would date the Cave Creek formation as pre-

Rhyolite Canyon.

3. The degree of alteration in the Cave Creek formation is more characteristic of the lower rhyolites.

4. Northeast and northwest trending faults cutting the formation and the 10* to 20° dips are characteristic of lower rhyolites

(terminology of Plate 1, pre-Rhyolite Canyon age) in the eastern

Chiricahua range.

5. Biotite-bearing welded tuffs in the Cave Creek form a­ tion are lithologically correlative with lower rhyolites in the H orse­ shoe Canyon area.

Horseshoe Canyon Area

The Horseshoe Canyon area is an excellent area for study­ ing the Tertiary volcanic sequence in the Chiricahua Mountains. The entire volcanic sequence, ranging from Cretaceous(?) andesites to

Quaternary basalts are exposed in steep canyon walls cut into antithetic fault blocks. The author spent many days in this area. 42

From well-exposed thick sections, the relative youth of the Rhyolite

Canyon ash-flow sheet was verified and distinguishing characteristics of older ash-flow sheets were determined, This in turn provided

evidence for the age of m ajor sequences of ash-flow deposits in the

Cave Creek area and in the southern Peloncillo Mountains. In

addition, the sequence of post-Rhyolite Canyon rocks provided

correlation of events outside the caldera with those inside the caldera.

The stratigraphy of the Horseshoe Canyon area is sum­

marized as follows:

Lower Rhyolites

Only a portion of the lower rhyolites are included in any one

section exposed (Figure 16). Their total thickness is estimated at

3000-4000 feet. The thickest exposure is in the spectacular fault

scarp forming the western wall of Horseshoe Canyon, where the

canyon trends north-south.

Unlike the Cave Creek area, lower rhyolites here consist

of a large number of lithologically distinct units. The welded tuffs

are moderately welded and separated by ash, fine-grained flows, and

tuffaceous sediments. Phenocrysts of 1-3 mm alkali feldspar and

quartz compose from 5 to 25 percent of the rock. Plagioclase may

be present. Biotite is usually present, ranging from 1 to 3 mm in 43

A Sample Rock Feet Map onit______^ no. type______D e s c r ip tio n

Planar, jointed cliffs, sim ilar in appearance to rh y o lite welded tuffs but without horizontal fractures. flow s Light to medium gray with sparse quartz and % sanidine (up to 0. 3 mm); essentially cryptocrys- u p p e r 1 0 0 0 - \ talline. Banded lavender at the base. r h y o lite s Thick (4 ft.) bedded; very fine tuff with less than r h y o lite 5 percent quartz and feldspar up to 1 mm; rare g p u m ic e o u s biotite; very small pumice lapilli. Lower part tu ff brecciated. 8 0 0 - s s / Thin to thick bedded, forming pale brown ledges; rh y . tu ff firm; light grayish brown to tan or pale orange; ?*s-? b r e c c i a lapilli and blocks of underlying Rhy. Can. welded o r a g g l. tuff m ost abundant. X flow Reddish brown rhyolite. 6 0 0 - / _ rh y . Ashy top containing 20 percent dark lithics (up to -467 welded 3 mm); crystal-rich, lithologically correlative | tu ff with tuff on Chiricahua Peak. tu ff 1 and Pale reddish brown, biotite-bearing; includes \ a g g l . pumiceous layer. 4 0 0 - C R h y o lite j Ashy top, rem ainder only slightly welded; C an y o n IB rh y o lite lithologically correlative with members 4 and/or B F o r m a tio n 1 w e ld ed 6; 1-2 mm phenocrysts. Moderately welded cliffs, densely welded tu ff toward base; basal 15 feet ashy; jointing is rh y o lite 2 0 0 - Base covered. 1 rh y o lite Moderately welded. Probably correlative poor, 3-6 ft. wide; slabby but irregularly w e ld e d w eld ed with members 4 and/or 6. fractured; containing less than 1 percent -474 ~~~ tu ff -510 tu ff lithics. Probably correlative with member s Basal lithophysae zone, 2 ft. wide. 4 and/or 6. 0- -194 \ t u f f Moderate brown with basal lapilli. a g g l. Grayish orange pink. v o lc a n ic Coarse with small subangular cobbles. — ^ a i r - f a l l Thin bedded, pale yellowish brown. w co n g l. Fine sandstone in lower part. w e ld e d Granitic weathering, crystal-rich; massive, Very thick (6 ft.) bedded with alternating / . fin e tu ff vapor-phase; contains biotite. 1 ashy pale orange and firm tan beds. / - 2 0 9 - V lo w e r p o rp h y - v o lc a n ic Fine to coarse; very pale orange; j r h y o lite s r itic Coarsely porphyritic. sa n d sto n e thin bedded. # • g ra n ite ( ? ) xSS^ flow Biotite-rich; biotite less than 1 mm. , w e ld e d Tan to light reddish brown; 15-30 per­ -4 0 0 — •556 tu ff cent phenocrysts, 1-2 mm at top and 2-3 mm near base; up to 1 mm biotite.

FIGURE 16 - - Reconnaissance sections in the Horseshoe Canyon area. L o c a tio n s: A- NW1/4 and SWl/4, SE1/4, sec. 17, T. 19S. , R. 31 E. B- Center of NE1/4, NEl/4, sec. 16, T.19S. , R. 31E. C- Center of SE1/4, sec. 15, T. 19S. , R.31E. 44 in size and amounting to 2 to 3 percent by volume. The presence of biotite and plagioclase implies a pre-Rhyolite Canyon age.

The inter stratified flows are patched or zoned with very light gray to pale red colors, indicating varying degrees of oxidation.

The sediments include thin bedded volcanic sandstone and conglomer­ a te .

A K-Ar age of 24. 5+1.2 m. y. (DM-1-68, Appendix II) was obtained on a sample from the underlying ash-flow sheet at the base of section B, Figure 16 (sample 556). This age is very close to the average of two ages obtained on the Rhyolite Canyon Formation

(24. 9 +.0. 6 m. y. ), which overlies it unconformably. These relations indicate that (1) the Rhyolite Canyon sheet followed the previous sheet within a very short time, and (2) faulting and northwest tilting (of the order of 20° ) took place between eruption of two ash- flow sheets. It is noted also that the underlying tuffs are lithologi­ cally correlative with tuffs in the W eatherby Canyon area (Appendix

I) and the tuff immediately underlying Rhyolite Canyon tuff at South

Antelope Pass (Table 15, Appendix I).

Above the unconformity and underlying Rhyolite Canyon tuff in section B (Figure 16) are several tuffaceous units including

air-fall tuffs. A ir-fall tuffs commonly precede ash flows; these tuffs may be genetically related to, and more appropriately mapped 45 with, Rhyolite Canyon tuff. Indeed the unconformity indicated in section B may be taken as the dividing line between lower rhyolites and Rhyolite Canyon tuff. It is noted in addition that sim ilar air- fall tuffs may be found at Antelope Pass (Table 15, Appendix I), immediately underlying Rhyolite Canyon tuff.

Rhyolite Canyon Formation

An estim ated 600 feet of Rhyolite Canyon tuff is exposed in one section in Horseshoe Canyon with the base covered (section A,

Figure 16). Lower portions of the formation are correlative with members 4 and/or 6 in the Monument on the basis of phenocryst content and size, and jointing patterns. On sim ilar grounds, the upper portion is probably correlative with member 8 in th e

Monument and the top of Chiricahua Peak (see Figure 50, Appendix

III). The Rhyolite Canyon sequence here is broken by a petrograph-

ically and lithologically anomalous tuffaceous unit, some 70 feet

thick, which suggests a cooling break. A sim ilar break is indicated

in the Monument section between members 6 an d 8.

At least 500 feet of Rhyolite Canyon tuff is exposed below

Mayday Peak on the east side of Price Canyon (see Plate 2 for

location). From the canyon road the ash flows can be seen forming

overlapping benches. Rhyolite Canyon tuffs are also exposed ' 46 immediately below the moat rhyolites just southeast of Sentinel

Peak (8600-foot level).

Throughout the Horseshoe Canyon area. Rhyolite Canyon tuffs are poorly to moderately welded, forming at least three distinct ash-flow units, separated by less welded zones. Such welding reversals are present between members 4, 6 an d 8 in the Monument.

Upper Rhyolites

More than 600 feet of rhyolite agglomerate or tuff breccia, pumiceous tuffs and aphan itic flows overlie the Rhyolite Canyon tuff

(section A, Figure 16). This sequence is essentially identical to the moat rhyolites in the caldera, which will be discussed later.

Medium gray and darker flows are associated with the

Horseshoe fault in Horseshoe Canyon. These flows are siliceous, oxidized, and very hard, different than the uppermost rhyolite flow

(section A, Figure 16). Their occurence is very probably related to the fault zone. They overlie Rhyolite Canyon tuff.

Shake Gulch Area

Rocks found in the Shake Gulch area include Rhyolite Canyon tuff and upper rhyolites. Older rocks are exposed at the base of the eastern slopes of this westward-dipping fault block (in Price Canyon) and in Red Rock Canyon. At least two distinct ash-flow units make up the Rhyolite Canyon sheet in this area. 47

The upper rhyolite sequence was not examined in its entirety. From the characteristics that were observed (Figure 17), it looks like it may be correlative with the moat rhyolites in the caldera, except that the flows which cap the section in the southern part of the area, are quartz latitic in composition, unlike any units within the caldera.

Flows capping Sage Peak are rhyolitic in composition, medium gray and thinly laminated. The area three-quarters of a mile north-northwest of Sage Peak, characterized by vertical flow structure, looks like it may be a source vent. The location of such a vent in what is believed to be right outside the caldera wall is surprising. If so, it could be an unusual occurrence in calderas, a case where lavas characteristic of the moat were extruded outside the caldera as well.

Reddish brown flows border the Shake Gulch area on the south, west, and north^ (Figure 18). Their distribution along the

3. These flows are mapped as upper rhyolite, Tru, on Plate 1, where the contacts indicate their extent along the border of the Shake Gulch area. 48

R o ck ty p e Description Map ______Thick, (ft. ) ______u n it q u a r tz Light brownish gray and very latite(? ) light gray laminae; 2-3% pheno- flo w s crysts (1mm) of feldspar andtgz; ___ 300____ plag exceeds K-feldpar______tuffaceous 200 Light gray upper

tu ffa c e o u s 200 Massive; pale light gray

tu ff Light gray to light tan bedded b r e c c ia tuffs at top. Main part contains o r a g g l. subangular sandstone blocks, up 300 to 1 ft. j coarser toward hnttom

rh y o lite 20% phenocrysts (up to 5 mm); 2% w e ld ed magnetite (ave. 0. 4'mm). Basal R h y o lite tu ff flow breccia with lapilli of C anyon 500 laminated rhyolite F m .

rhyolite(?) w e ld ed Grayish red with sparse biotite lo w e r tu ff r h y o lite s

>!c P la te 1.

FIGURE 17. --Generalized section in the Shake Gulch a r e a . Reconnaissance. 49

Sunset fault** and middle Rucker Canyon^ suggests that they were extruded along these zones. These flows are reddish-brown weathering, silicic and very hard. They are seen interbedded with the upper rhyolites in Red Rock Canyon.

Vertical displacement on the Sunset fault is estimated at a minimum of 3000 feet based on the stratigraphic discontinuity between

Cretaceous rocks on the up thrown side and the top of the Rhyolite

Canyon sequence on the downthrown side. A vertical displacement of

the same magnitude is estimated for the Horseshoe fault which bears

the additional sim ilarity of having reddish brown silicic flows border­

ing or covering the fault zone.

Bruno Canyon Area

The complete Tertiary sequence is well exposed in the Bruno

Canyon area especially in the south wall of lower Rucker Canyon.

That portion from lower Rucker Canyon to Hunt Canyon (Plate 1) is a

"model" basin and range fault block. Its east-west dimension is about

4. The names of Sunset, Cholla, and Rucker faults are used following Epis (1956).

5. For the purpose of proper location, Rucker Canyon is divided into three segments, upper, middle, and lower. Upper refers to the south-southeast-draining portion from Chiricahua Peak toward Sage Peak. Middle refers to west-southwest-draining portion passing through Rucker Lake. Lower refers to the west-draining portion at the mouth of the canyon. 50

5 miles, its north-south dimension, about 8 miles. Major faults bound the block on the east; faults on the east side are covered by younger deposits. As evidenced by angular unconformities, this block has been tilted westward in increm ents of 10° -15° following deposition of the lower rhyolites and again following deposition of the older alluvium (section KL, Plate 1) which conformably (or nearly so) overlies the Rhyolite Canyon ash-flow sheet.

Epis (1956) mapped the Tertiary volcanics in this area as a unit which he called the Chiricahua rhyolite. This includes the author's

"lower rhyolites" and Rhyolite Canyon Formation. The author con­ fined his work to mapping the contact between these two units and to

examination of the Rhyolite Canyon tuffs.

A description of the lower rhyolites, based prim arily on

Epis' description is included for completeness. The lower rhyolites

consist of a large number of distinct units of varied lithology, color

and thickness (Figure 19). Maximum thickness is estimated at

around 3000 feet between Bruno and Rucker Canyon. Thickness of

individual units ranges from inches to 150 feet. The rocks are

predominantly pyroclastic, ranging from ash, lapilli tuffs, tuff breccia

and welded tuff with subordinate interstratified flows. Lapilli tuffs

are the most common. Quartz is ubiquitous and sodic plagioclase 51

FIGURE 19. - - View of lower rhyolites (A) and Rhyolite Canyon tuff (B) in North Bruno Canyon. The former are light brown to pale orange, pre­ dominantly lithic tuffs; the latter are pale brown, moderately welded tuffs. 52 generally subordinate, indicating rhyolitic to quartz latitic com­ position. Biotite is a common accessory.

The Rhyolite Canyon sheet generally thickens to the north in this area. In north Bruno Canyon the thickness is estimated at a minimum of 1500 feet. An appreciable thickness in Tom Ketchum

Canyon, around 400 feet, makes it seem likely that Rhyolite Canyon tuff extends even south of the area mapped (Plate 1).

The Rhyolite Canyon sequence was compared at two locations in the area: below Bruno Peak on its south side and in

Tom Ketchum Canyon (see Plate 2 for location), 8 miles south of

Bruno Peak. It was noted that the Bruno Peak section (1) is consider­ ably thicker, (2) contains a higher percentage of phenocrysts,

(3) contains a greater proportion of densely welded tuff, (4) has more units, and (5) has more pumice and lithic inclusions. These differences are what one would expect if Bruno Peak is closer to the source area.

The older alluvium in the Pedregosa Mountains is the Mesa

Draw formation of Epis (1956). Epis describes it as consisting of a

series of medium to coarse grained, poorly consolidated volcanic

sandstones with subordinate interbedded coarser and finer clastic layers. Epiclastic in nature, it consists of angular lithic and

crystal fragments derived mainly from the underlying volcanics. 53

Total thickness, according to Epis, does not exceed 1500 feet in the valley between the Pedregosa and Swisshelm Mountains. The log of a well located just north of the entrance to Rucker Canyon probably describes a section of this formation (Table 4).

The lack of significant angular discordance between the

Rhyolite Canyon ash-flow sheet and the older alluvium suggests that the alluvium is not much younger.

TABLE 4. - -Log of Well in Older Alluvium

Located north of entrance to Rucker Canyon in the SW1/4, sec. 15, T.19S. , R.28E. From files of the University of Arizona., Department of Hydrology.

Description Depth (ft.)

boulders in clay 210 tight sedimentary fm. 240 boulders in clay 400 gravelly formation 450 tight sedimentary fm. 500 small boulders, gravelly fm. 660 fine tnesh water sand 670 hard fine mesh water sand 1050 hard fine mesh water sand 1100 total depth 54

Turkey Creek Caldera

The -Turkey Creek caldera is a resurgent caldera of the type

Valles (Smith, Bailey, and Ross, 1961). It is named after the major stream channel which drains it. The diameter of this structure, estimated from the outermost ring fractures, is about 13 miles.

Approximate outline of the caldera is indicated on the relief map

(Figure 3) and geologic map (Plate 1).

Recognition of this structure as a caldera is based upon the sim ilarity of its features to those characterizing more unequi­ vocal calderas such as the Valles caldera (Smith, Bailey, and

Ross, 1961), Creede Caldera (Ratt6 and Steven, 1967; Steven and

Ratte, 1965) and the Timber Mountain caldera (Christiansen et al,

1965). These include: (1) several features of the rocks present: types, relative abundance, order of emplacement, distribution, and deformation, and (2) structure: ring faults, dome, and stratigraphic discontinuities along the wall.

Physiographically, the Turkey Creek caldera is character­ ized by a central erosional cirque or corrie^ surrounded by a high- standing annulus of resistant rhyolites. The cirque is about 6 miles

6. Interestingly, this is an erosion caldera of the type Caldera of La Palma from which Lyell derived the term "caldera" for more general application (Cotton, 1944, p. 390). In order to avoid confusion, however, it is referred to as an erosional cirque, or simply, a c irq u e . 55 across and is walled in for more than 270*by steep escarpments and

slopes. Internally, drainage follows centrifugal courses to a few

major outlets which drain westward to the valley (Figure 20). Ex­

ternally, drainage is generally radial until beyond the resistant m oat rhyolites.

It is apparent that an inversion of topography has been

effected in the Turkey Creek caldera, due to different erodability

of rocks in structural zones of the caldera (Figure 21). In the moat,

great thicknesses of silicic, highly resistant lavas remain high, while

in the dome welded tuffs, fractured and altered by intrusion, have

been efficiently removed, forming the cirque-like structure. The

floor of the cirque lies some 2400 feet below its rim . Similarly,

welded tuffs and older volcanics forming the caldera walls have been

largely removed. In places the volcanic cover has been completely

removed along with unknown thicknesses of the less resistant

Cretaceous sediments so that at present, a part of the caldera wall

is as much as 3000 feet below the moat.

The caldera stratigraphy will be described in detail in the

following chapter. Only a brief introduction is made at this time.

Rocks exposed in the caldera are subdivided into the

following major units, in order of decreasing age: Rhyolite Canyon

tuff, monzonite porphyry, and moat rhyolites. Rhyolite Canyon tuff, 56

109*15*

5 MILES

31*45"

FIGURE 20. --Drainage pattern in the caldera. The rim of the central erosional cirque (indicated by dashed lines) separates the internal drainage pattern from the radial pattern off the dome. The cal­ dera wall is indicated by a solid line. 57

e r o s io n a l c irq u e w a ll d o m e m o a t

FIGURE 21. --Schematic cross-section of caldera showing structural features. Dotted section has been removed by erosion.

up to 3000 feet thick, dips radially 10° to 25°off the central dome.

A fine grained monzonite porphyry is exposed in the floor of the cirque. The moat contains more than 1000 feet of breccia, fine pumiceous tuffs and ash, and capping resistant rhyolite flows. In addition, the same monzonite found in the dome covers major portions of the moat where it is at least 2000 feet thick.

With the exception of the monzonite, rocks of the caldera sequence are closely related in petrography and chemistry. Homo­ geneity and variations within the sequence will be discussed in

Chapter VI.

The outermost ring faults are exposed along the caldera wall, in one case perhaps for as much as 70* of arc. Although the objective of this research did not require extensive structural 58 studies, quite a bit of structural data was accumulated. Structural features are mentioned throughout the discussions of stratigraphy in Chapters IV and V and will be discussed further in a chapter on structure, Chapter VII.

The Distribution of Rhyolite Canyon Formation in the

Chiricahua and Northern Pedregosa Mountains-A Summary

In the Chiricahua and northern Pedregosa Mountains,

Rhyolite Canyon tuff makes up a major part of the Tertiary volcanic sequence with two notable exceptions. The first is in the area of

Cochise Head and Maverick Peak, northeast of the Monument. The second is in the Cave Creek area. In the first case, absence of

Rhyolite Canyon tuff may be explained by unfavorable topography at the time of deposition. This is considered very likely considering the rapid thinning of some members of the Rhyolite Canyon Formation toward the north in the Monument (Enlows, 1955). The author also noted a pronounced northward thinning of older ash-flow deposits in the Chiricahua range. For example, in the Bruno Canyon-Rucker

Canyon area, about 3000 feet of older ash-flow deposits are present while in the Monument only about 200 feet are present (Figure 8).

Notably, flows, breccia and sediments form the rem ainder of the lower rhyolites in the Monument (Faraway Ranch Formation),

indicating that this was a region of alluvial sedimentation and volcanic 59 activity. Northward thinning of Tertiary rocks and sedimentation in the Monument indicate Tertiary deposition in the Chiricahua

Mountains against a pre-existing (Laramide ? ) high to the north with a relief of the order of thousands of feet.

In the case of the Cave Creek area, at least some Rhyolite

Canyon tuff probably covered the entire area (and may still be found in unmapped areas). It probably never was very thick here consider­ ing the tremendous pile of volcanics already deposited in the area

(some 4, 500 feet remain at present with top eroded). Another possible explanation is that a topographic barrier existed between the caldera and Cave Creek. This is suggested by the rapid thinning of lower members of the Cave Creek Formation, (about 3000 feet in less than 5 miles) over the Cretaceous to the southwest along South Fork.

An estim ate of the areal extent of the Rhyolite Canyon

Formation can be made by drawing a convex curve around the presently known outcrop area, including those in the Peloncillo and

Swisshelm Mountains (Figure 5). Seven hundred square miles is obtained. If only the Chiricahua and Pedregosa Mountains are included, 400 square miles is obtained.

Estimated thicknesses in the various areas described above are tabulated in Table 5. Taking 3000 feet as the average thickness in the caldera, and 800 feet outside of the caldera, the original i

t i 60

TABLE 5. --Thickness of Rhyolite Canyon Tuff in the Chiricahua and • Northern Pedregosa Mountains ■

Area Location Est. thick, N o. u n its (dist. fr. caldera wall) ( f t . ) id e n tifie d

M o n u m en t- Sugarloaf Peak 1700 7 Cochise Head (3 m i. n o rth ) *’

Horseshoe Canyon Horseshoe Canyon 600* 3 (4 mi. east-southeast) j'

Shake Gulch Red Rock Canyon 300 3 (2 mi. south-southeast) |

Bruno Canyon North Bruno Canyon 1500 2 - (4 mi. south-southeast)

Bruno Canyon Tom Ketchum Canyon 400 1 (12 mi. south) ' i,. Turkey Creek 3000 2 C a ld e ra

* Base not exposed. volume of Rhyolite Canyon tuff in the Chiricahua and northern Pedre­ gosa Mountains is 110 cubic miles (450 cubic kilom eters). When the

Peloncillo and Swisshelm Mountains are included and 400 feet taken as the average thickness in the valleys, the volume is 130 cubic miles

(530 cubic kilometers). This exceeds in volume the deposits in the

Aso and Valles ash-flow fields (Smith, I960, p. 819). C H A P T E R V

STRATIGRAPHY IN THE TURKEY CREEK CALDERA

For purposes of discussion, the caldera is subdivided into four sectors as indicated in the index map in Plate 1. These are structural-stratigraphic subdivisions and they are indicated by

Roman numerals.

Rocks Forming the Caldera Walls

Rocks of various ages are exposed in the caldera walls. *

These rocks are discussed briefly because of their importance in arriving at the conclusion that major stratigraphic discontinuities exist at the caldera wall. In sector I, andesites of probable

Cretaceous age are found at the entrance to Witch Canyon (Plate 1) in proximity to the moat rhyolites. Also along the northern wall, a rhyodacite flow (lithologically correlative with member 7 of the

Faraway Ranch Formation) is found near the entrance to Pine and

Pinery Canyons. The pre-Rhyolite Canyon age of this rock is

1. The term "wall" is used throughout the text in a struc­ tural sense. In actuality, a wall in the topographic sense is nowhere present, since rocks forming the original wall have been eroded to a depth as low or lower than that of the moat.

61 6 2 verified since it is overlain by Rhyolite Canyon tuff. This rock is

notably oxidized in the vicinity of east-w est faults which are probably

part of the ring-fracture system. A few miles east of this on the

south wall of Pinery Canyon is a rock whose mineralogy implies a 2 correlation with a unit in the lower rhyolites.

In sector II, lower Cretaceous limestones, sandstones,

and conglomerates form the wall rock. These rocks are hydrotermally

altered in the ring-fracture zone. In places they are overlain by

rhyolites whose positive identification is obscured by oxidation (red­

dening) and crystallization.

The location of the caldera wall in sector III is obscured

by the overlying moat rhyolite flows. In sector IV, Cretaceous

sediments again form the wall rock for a couple of miles where the

ring fracture transects the Rucker horst block. West of this,

lower rhyolites form the wall rock making a contact with the moat

monzonite that dips something like 30° inward.

2. Members 1 or 2 of the Faraway Ranch Formation.

3. The Rucker horst block is the area between the Shake Gulch and Bruno Canyon areas, bounded by the Cholla and Sunset faults (Plate 1). 63

Rhyolite Canyon Tuffs

Rhyolite Canyon tuffs are the oldest known rocks exposed in the caldera. They are found (1) exposed in the steep slopes and scarps forming the walls of the central cirque, dipping 25* -35° subradial to the dome, (2) in isolated patches lying on the monzonite porphyry, (3) in a central "pendant, 11 the area in the northeast portion corner of section IV (nearly central to the caldera) where they are surrounded on three sides by monzonite, and (4) fairly flat-lying in the moat south-southeast of the pendant.

T h ic k n e ss

Thickness estim ates of Rhyolite Canyon tuff in the caldera are summarized in Table 6. Three thousand feet is considered to be a reasonable figure for the maximum thickness presently exposed.

Petrography

Welded tuffs in the caldera are correlative with the Rhyolite

Canyon tuff in the Monument on the basis of the distinguishing characteristics discussed in Chapter II, including macroscopic, microscopic, chemical, and isotopic (K-Ar age) characteristics.

They differ from tuffs in the Monument in having, on the average, a greater percentage of phenocrysts, xenoliths, and pumice, and in being slightly more densely welded. These differences are what 64

TABLE 6. - -Estim ated Thickness of Rhyolite Canyon Tuff in the C a ld e ra

Section Thick, (ft.)

Thickest subvertical section, east side 2000* of central "pendant" in Polebridge Can.

Measured from geologic sections (Plate 1): Rattlesnake Peak, section E-F 2850 Round Park, section G-H 3000 Chiricahua Peak, section I-J 3900 Rock Peak, section A-B 3000

* Top eroded.

would be expected from lateral changes in ash flows, considering

that the Monument is a couple miles from the caldera. In addition,

welded tuffs in the caldera are commonly crystallized, oxidized, or

decomposed to varying degrees, as a consequence of intrusion of

the monzonite. Characteristics near an intrusive contact are

considered in a separate subsection.

Phenocryst content is nominally 25 percent, but ranges

from 5 to 35 percent. Sanidine and quartz are nominally 2-3 mm in

size, and as large as 4 mm. ‘ Magnetite is generally more abundant

than in the Monument, amounting to more than 6 percent of the

rock and up to several mm in size. Xenoliths are sparse in upper

portions but amount to 30 percent in lower portions. Pumice blocks 65 are larger than anywhere outside the caldera, sometimes exceeding

15 cm in length. They are distinct by their medium gray color compared with light gray outside the caldera. Microscopic texture of unaltered welded tuffs is typical of the Rhyolite Canyon tuff

(Figure 22).

Upper and Lower Units: Characteristics and Distribution

For purposes of discussion Rhyolite Canyon tuff in the caldera is divided into two units. These are referred to only as the upper and lower units. These may or may not be mapable units throughout the caldera.

The prim ary distinction between these units is xenolith

content. Other distinguishing characteristics include welding,

weathering, color, and phenocryst content. The lower unit contains up

to 30 percent dark angular lithic fragments, 2 to 3 cm in size,

although some parts are relatively free of lithics. It is generally

moderately to densely welded, a slope-form er, and exhibiting pale

red, pale or dark reddish brown, and brownish gray colors which

may grade from one to another within a few hundred feet vertically.

The lower unit also consists of several distinct rock units. The

upper unit on the other hand generally contains less than a few

percent lithics and their size does not exceed 1/2 cm. It is more

densely welded, forming light gray cliffs at a distance (such as 66

FIGURE 22. - -Photomicrographs of Rhyolite Canyon tuff in the caldera. Showing flattened and welded shards. Phenocrysts are sanidine, quartz, and magnetite. Sample is from upper part of section in the caldera. Nicols: left, uncrossed; right, crossed. X40.

Chiricahua Peak viewed from the south or southwest). In hand sample it is typically varicolored with very light gray and moderate pink streaks on a light medium gray matrix. In addition, there is a tendency for this unit to have more magnetite. One final characteris­ tic which helps distinguish these units is alteration. The lower unit is obviously more susceptible to alteration by the intrusive monzonite porphyry and usually is altered to some degree.

A 600-foot section below Rattlesnake Peak (Table 7) illustrates the different characteristics of the two units. The 67

TABLE 7. --Partial Section of Rhyolite Canyon Tuff below Rattlesnake Peak

South side of peak, in Rattlesnake Canyon.

Description Thick, (ft. )*

Moderately welded, cliff-forming; light 100 medium gray with light gray and moderate pink streaks; black vitrophyre at base; u p p e r 8-inch pumice lenses; 30 percent fresh u n it sanidine and quartz (nominally 2-3mm); several percent magnetite (up to 2mm); shards, in matrix. Top eroded.

Slope-former, probably consisting of 500 several units; pumice sparse; inclusions of dark aphanitic volcanic fragments (l-3mm), amount to as much as 30 percent; 20-30 percent phenocrysts, (nominally 1- lo w e r 3mm, but as large as 5mm); several u n it percent magnetite (to 2mm); sanidine considerably more altered than in the upper unit, being whitish; quartz is corro­ ded; shards are only barely distinguishable. Base covered.

* t30 percent. 68 vitrophyre at the base of the upper unit is evidence for a cooling break in the sequence.

A 2000-foot section on the east side of the central "pendant", just northwest of Polebridge Canyon, contains both upper and lower units. About 1000 feet of the upper unit are exposed in the magnificent cliffs on the south side of Chiricahua Peak (sector III). About midway in this exposure is a more easily weathered, less welded zone, indicating compound rather than simple cooling. The extremely variable thickness of the upper unit is indicated by a thickness of only

100-200 feet on the south side of Rock Peak (sector I).

Positive identification of tuffs in substantial portions of the caldera was not possible because of alteration. In some cases it was not possible to decide even whether the rocks are welded tuffs or flows. A few comments are made on these correlation p ro b le m s .

In Stanford Canyon (sector IV, Plate 1), just north of the intrusive contact with the monzonite, the tuffs are probably, the lower unit of Rhyolite Canyon tuff on the basis of xenolith content and number of units of different lithology. Pumice lenses are anomalously lengthened and irregular suggesting movement as a fluid following compaction. 69

Rocks below Black Mountain look like welded tuffs in that they tend to form vertical columns and have shards in the m atrix but flow lamination may also be observed. The size, relative percentage, and degree of fragmentation of phenocrysts are consistent with a Rhyolite Canyon correlation.

Rhyolite Canyon tuff in the Turtle Mountain block contains up to 10 percent xenoliths and is probably the lower unit. Correlation of rocks in the western part of the caldera from Rock Creek (sector I) to the entrance to Stanford Canyon is indefinite because of alteration.

Little can be said at this point regarding the correlation of

separate members of the Rhyolite Canyon tuff between the Monument

and caldera. The absence of members 1 and 2 from the caldera

seems likely because no tuffs have been found in the caldera which

contain either biotite or hornblende. Data on relative proportion of

phenocrysts favors the correlation of the upper unit in the caldera

sequence with member 8 in the Monument (Appendix III).

Regarding the location of ash-flow source vents in the

caldera, although they were probably obscured by subsequent

intrusive and volcanic activity, in the Turkey Creek caldera, flow

characteristics and lateral variations could be used to determine at

least the general location of the vents. This may be possible because

the tuffs are exposed in the caldera itself. 70

Characteristics near Contact with the Monzonite

Contacts between the welded tuffs and monzonite porphyry are intrusive in nature. The tuffs have been altered and crystallized with increased intensity toward the contact. The width of the alteration zone in the tuffs is a function of contact attitude. In steep

contacts, visible effects are noted for a distance of 300 to 500

feet. In horizontal contacts, the welded tuffs show visible alteration

for as much as 1000 feet above the contact.

A progressive change in macroscopic and microscopic

characteristics is noted in the tuffs as the contact is approached.

Recognition of these characteristics and their change toward the

contact is helpful in distinguishing between the crystallized welded

tuff and the monzonite, and in determining proximity to the contact.

The problem of alteration of the welded tuffs is ubiquitous in the

c a ld e r a .

The following illustrates the changes that take place as a

vertical contact is approached:

1. At about 500 feet from the contact the color is normal.

Outlines of pumice lenses and xenoliths are normal. Vertical

fractures are more closely spaced, on the order of 10 feet, with

preferential alignment parallel to the contact. Quartz phenocrysts

are normal, sanidine is pale and slightly whitened. M icroscopically, 71 the matrix is composted of m icrolites; shards are no longer distin­ guishable; and minute iron ore is disseminated throughout, locally forming compact aggregate grains. The groundmass may be brownish. Quartz is corroded and sanidine is sericitized along edges and internal cleavages (Figure 23A). Some sanidine appears to have been separated into isolated parts.

2. At about 50 to 100 feet the rock becomes lighter in

color due to crystallization of the groundmass. Color ranges from

a varicolored light olive gray with pinkish gray altered sanidine and

clear quartz. Some silicification may be found along fractures.

Microscopically, the m atrix is composed of m icrolites. Corrosion

of quartz and sericitization of sanidine is pronounced (Figure 23B).

3. Within a few feet or few tens of feet of the contact, the

color is light gray or white and flow banding from remobilization

of the tuff can be seen. Texturally the crystallized welded tuff and

intrusive rock are indistinguishable. Only the quartz phenocrysts,

seen to within a fraction of an inch of the contact, allow one to

distinguish the two rocks. Microscopically, the m atrix is composed

of intergrowths of quartz and alkali feldspar m icrolites, highly

elongate to equant (Figure 23C). Crystallization of the groundmass

(0. 2 to 0.4 mm size crystals) is the same as the intrusive. 72

FIGURE 23. - -Photomicrographs of welded tuff showing changes as contact is approached. At about 500 feet from the contact (A), there is corrosion of quartz(l), decomposition of sanidine (2), and the matrix no longer looks like a welded tuff. At about 100 feet from the contact (B), the matrix is crystallized and the corrosion of quartz (3) and sanidine (4) is pronounced. At only a few feet from the contact (C) only skeletal outlines of the phenocrysts are seen and the matrix consists of intergrown microlites of quartz and feldspar. Nicols: left, uncrossed; right, crossed. X40. 73

FIGURE 23. - - (Cont. ) Nicols top, uncrossed; bottom, crossed. X40.

V

>>’ 74

Low angle contacts may show evidence of fluidization

(Figure 24) and for as much as 1000 feet from the contact supergene deposits of iron and manganese oxides, a fraction of a m ilim eter thick, line the fractures.

Porphyritic Rhyolites

The porphyritic rhyolites are found only in the vicinity of

Stanford Peak in the moat (sector IV). Total thickness is not known.

Lithologically the porphyritic rhyolites are indistinguisable from

partially crystallized Rhyolite Canyon tuff near a contact with the

monzonite. Microscopically, however, the following differences

warrant their separate classification:

1. Phenocrysts are fresh in the porphyritic rhyolites. This

would not be the case if they were crystallized tuff.

2. Phenocrysts are mostly larger than 0. 7 mm; there is not

the abundance of small crystal fragments that there is in the welded

tu f f s .

3. Flow lamination is much more pronounced than observed

in welded tuffs remobilized near a contact.

Vertical flow structure on Stanford Peak suggest that it may

be a source vent for this unit. The age is uncertain. A post-Rhyolite

Canyon age is suspected considering its topographic position above

Rhyolite Canyon tuffs. 75

Moat Rhyolites

General Characteristics and Thickness

The moat rhyolites^ are a sequence of rhyolitic rocks of varied texture and origin confined for the most part to the caldera moat. They include the following rock types, in approximate order of decreasing age: flows, tuff breccia and tuffaceous sediments, air- fall tuffs, fine pumiceous tuffs, and flows. The tuffs and flows are by far the most abundant of the rocks exposed. With minor exception all these rocks are petrographically and chemically very sim ilar to the

Rhyolite Canyon tuff (this will be discussed in Chapter VI).

Moat rhyolites are found throughout the moat but are the dominant unit in only sectors I and III, which together cover about

180°of arc. In these areas they cover an area about 3 miles wide.

In sector III the moat rhyolites extend beyond the caldera wall, concealing its precise location.

4. Use of the term "moat rhyolites" in this way is not com­ pletely satisfactory in view of the structural connotation. In particular, it is noted that these rocks extend beyond the wall. In addition, caldera fill may be included. A suitable textural or genetic name could not be applied to these rocks because of their varied nature and mode of emplacement. It should be noted also that two other map units are found in the moat: the welded tuffs and monzonite porphyry. These form only a lesser portion of and are not restricted to the moat, however. 76

The thickest section of moat rhyolites exposed is estimated at about 1700 feet (Table 8; Figure 25). Since the base is nowhere

TABLE 8. - -Estim ated Maximum Thickness of Moat Rhyolites E x p o se d

Top eroded and base not exposed.

Location Sector* Thick, (ft.)

Pine Canyon I 1200

Upper Rucker Canyon, III 1700 below Lone Juniper Spring

Monte Vista Peak, III 1700 on the south side

* Caldera sectors are indicated on an index map on Plate 1. exposed in the deepest parts of the moat and the top is eroded, no estimate of the maximum thickness can be made at this point. When all the subunits are mapped in the moat however, it should be possible to determine this. An initial maximum thickness of 3000 feet is considered reasonable based upon present knowledge.

Moat rhyolites dip typically about 10° radially off the dome^ where they overlie Rhyolite Canyon tuff with marked angular

5 . To what extent the attitude of these moat deposits is pri­ mary is a very interesting question which cannot be answered at this time. The question of whether doming continued throughout the period of deposition of moat rhyolites is of great interest in caldera research. 77

FIGURE 24. - -B recciation and fluidization of Rhyolite Canyon tuff near an intrusive contact. White specks in tuff (above pen) are altered sanidine phen o cry sts. Pen is 5 in. long. Location: below Ida Peak on its southwest side.

FIGURE 25. - - View of m oat rhyolites in upper Rucker Canyon. Rocks seen are predominantly rhyolite flows and tuffs composing the moat deposits. Some units are reddish brown in color (A). Looking north-northwest towards Chiricahua Peak (B). 78 unconformity. Because of this unconformity, the total thickness

(with the exception of the uppermost flow which is eroded at the top) increases radially from about 100 feet (on the wall of the cirque) to around 1000 feet within about 2 miles (1 mile inside the caldera wall). This means an angular unconformity of about 5 ° .

For purposes of further description, the moat rhyolites are subdivided into four main subunits: lower flows, tuff breccia and tuffaceous sediments, fine tuffs, and upper flows. A generalized section in the northern part (sector I) is described in Figure 26 and a section in the southeastern part (sector III) in Table 9.

Lower Flows

Finely porphyritic rhyolite flows form the base of the

section exposed in Pine Canyon (location D, Figure 26) and Rattle­

snake Canyon (location E, Figure 26). In the form er, the tuff breccia and sediments are absent, and fine tuffs overlie the slightly

eroded, hydrated top of a glassy flow which thins to the west as

though it originated from near Dawings Pass. The erosional surface

is significant since it indicates a hiatus in the eruption of the moat

thyolites. In the Rattlesnake Canyon section a finely porphyritic

rhyolite(? ) bearing a few percent biotite forms the base. The

presence of appreciable biotite makes this rock anomalous among

rocks in the caldera. 79

Description

Cliff-forming, vertically jointed; forming tall II pillars more rugged than welded tuffs; subtly cav­ ernous and sheeted on a scale of cm. ; light to flow s medium gray with white ashy laminae. Basal "0—550 40 ft. is cavernous weathering and auto-brecci- ated; lowermost few feet is banded. .

Interbedded vitric and pumiceous tuffs containing less than 1 percent fragments of quartz, K-feldspar and an occasional biotite or magnetite crystal (less than 1 mm in size).

fin e Vitric tuffs form 20-200 ft. thick beds, grading tu ffs from light brownish gray ledges to pale red vitro- phyre with dusky brown lenses and sparse inclu­ 0 -3 5 0 sions; top is vesicular; base is pale pink ash. Pumiceous tuffs forming 20-40 foot thick beds that are ledge-forming in spite of their low density; pinkish gray to very pale orange, bearing more than 30 percent pumice; base is ashy.

a • tu ff . T a -~* ^ 1 A b r e c c ia Coarse layers are made up of lapilli and fine >-»— • - A & s e d i­ blocks of crystal-rich Rhyolite Canyon welded m e n ts tuff and a m atrix of crystal and glass fragments. t i - ' zy- 0 -3 0 0 flow s Flows are aphanitic rhyolite sim ilar to uppermost o r R hy. flows and biotite-bearing quartz latite. Rhyolite C an . F m . Canyon tuff is crystal-rich, like upper member.

FIGURE 26. --Generalized section of moat rhyolites in the northern part of the caldera. Based on sections at the following locations: A- south side of Rock Peak; NE1/4, sec. 33, T. 17S., R. 29E. B- south wall of Rock Can. ; NW1/4, SW1/4, sec. 6 , T.18S. ,R.29E. C- south wall of Pinery Can. ; just south of SE1/4, sec. 12, T. 17S. , R. 29E. D- Pine Can. ; 1 mile west-northwest of Dawings Pass. E- west wall of Rattlesnake Can. ; north-northwest of Rattlesnake P e a k . 80

TABLE 9. --Partial Section of Moat Rhyolites in Upper Rucker Canyon

Section exposed below Snow shed Peak.

R o ck u n it Description Feet*

flow s (s) Light to medium gray; laminated on a scale 350 of 5 mm thickness; less than a few percent phenocrysts of quartz and sanidine. Top e ro d e d .

o b sid ia n Black, perlitic, and in places, yellowish 300 flow gray; 5 % crystals (less than 1 mm in size) of quartz, feldspar, and pyroxene with a trace of plagioclase. Base is grayish orange with pale brown lenses.

tu ff Nonsorted, firm ly indurated unit made up of 300 b r e c c ia subangular lapilli, some blocks, and a m atrix of very small angular framents; lithics are predominantly crystal-rich Rhyolite Canyon welded tuff; microscopically, very fine shards may be seen in the matrix.

flow Pinkish-tan, laminated flows containing 300 reddish brown pisolites in the upper part.

flow s A number of flows of varied lithology, mostly 650 aphanitic; an upper flow is light gray to pinkish gray and finely laminated; flows in canyon bottom are reddish brown to gray and massive; containing spherulitic, piso- litic, and brecciated zones. Base covered.

* +. 30 percent. 81

Tuff Breccia and Tuffaceous Sediments

Rocks in this subunit are extremely variable in nature.

The objective of this research did not perm it studying them in sufficient detail to be able to classify them as to their origin. At this point the following types would be identified: tuff breccia, tuffaceous sandstone and conglomerate, and air-fall tuffs. In some exposures, classification was equivocal between more than one type.

One characteristic seems to be fairly universal in this sub-unit, that is, the presence of fragments of Rhyolite Canyon welded tuff. These fragments may be subrounded to angular and range from m illim eters to tens of feet in size and usually crystal-rich, densely welded, and pale red. The latter characteristics suggest correlation with an upper member of the formation, probably member 8. Fragments of lower members are also present but generally much less abundant.

The m atrix of the tuff breccia and sediments is composed of glass shards and crystal fragments or very fine silty m aterial.

The tuff breccia and sediments are found throughout the moat.

Thick sections are exposed in sectors I and III. Total thickness is not known to exceed 300 to 500 feet. An angular unconformity within this subunit is noted in Rucker Canyon (Figure 27).

In the section below Rock Peak (location A, Figure 26),

subrounded fine blocks are included in a tuffaceous m atrix which 82

FIGURE 27. - - Moat sedim ents (A) and rhyolite flows (B) in Rucker Canyon. Showing un­ conformities in sediments dipping toward the caldera. Probably close to caldera wall. Looking north from Sage Peak.

includes crystal fragments and glass shards (Figure 28). There is some evidence of sorting as coarser fragments are concentrated toward the base where they constitute up to 50 percent of the rock.

Considering the microscopic character of the rock and its firm induration, it seems that this rock could have formed as a consequence of the mobilization of talus or colluvium by hot tuffaceous material.

Alternatively, the blocks could have been erupted along with the tuffaceous material as an explosion breccia, the blocks rounded slight­

ly by abrasion. In any case the degree of sorting is consistent with movement over a relatively short distance. 83

FIGURE 28. --Tuff b reccia of the moat sequence below Rock Peak. Loose block (A) and well-indurated ledge (B). 84

In the Rock Canyon section (sector I; location B, Figure

26) these rocks weather with an appearance of bedding, 3 to 8 feet thick, but without noticeable sorting. Subrounded blocks are found up to 12 feet across. The m atrix is made up of angular to subangular unsorted fragments of quartz, sanidine, and magnetite and a fine silty material. Locally, thin fine grained lenses are also found. The fine grained m aterial could have resulted from the abrasion of moving blocks. The firm induration of this rock again suggests either mobilization by tuffaceous m aterial or an explosive origin although the action of water cannot be ruled out.

The blocks could have originated either from the dome or caldera walls. These deposits could represent avalanche deposits, either caldera or moat fill. In either case, the m aterial would not have had to be transported more than a few m iles.

In Rattlesnake Canyon (location E, Figure 26 ; also on the southwest slopes of Ida Peak in sector II), thin bedded lapilli tuffs are found (Figure 29). Some portions contain abundant lapilli and occasional blocks of Rhyolite Canyon welded tuffs. From a distance, other parts have the appearance of air-fall tuffs (Figure 29).

F in e T u ffs

This subunit includes several very thick beds of vitric and pumiceous tuffs (Figure 30). A ir-fall tuffs may also be found. 85

FIGURE 29. - -Bedded tuffaceous rocks exposed in Rattlesnake Canyon. Upper: 100-foot cliffs in east wall of canyon. Lower: thin bedded rocks in west wall. 86

Total thickness exposed exceeds 300 feet. A description of these rocks is given in Figure 26. They are petrographically identical to the overlying lavas and may be interpreted as composing the "tuff ring" or "pumice cone" which commonly precedes rhyolite flows.

Upper Flows

The uppermost rhyolite flows (Figure 26 and Table 9) are responsible for the high relief of the moat in sectors I and III, form ­ ing a resistant capping over the tuffs (Figure 31). These rocks form the rugged light to medium gray spires in the south wall of Pinery

Canyon (sector I). Even from a distance they are easily distinguished from Rhyolite Canyon welded tuff by the absence of parallel joints and horizontal partings. They form smooth or cavernous cliffs depending on the nature of flow laminae. Lamination may be very outstanding

(Figure 32).

Microscopically, in the northern part of the caldera these flows contain less than 1 percent sanidine (up to 0. 7 mm in size),

sparse biotite (0. 02 mm) and 1 to 2 percent opaques (up to 0. 07 mm).

M icrolites are abundant but not inconsistent with the extrusive '

classification. Flows in the southeastern part of the moat (sector ITT)

have the following characteristics distinguishing them from flows

in the northern part: (1) phenocrysts are twice as abundant; (2) quartz FIGURE 30. - -Rhyolite tuffs of the moat sequence. Very thick bedded, colored pale moderate orange. Just east of Rock Peak looking north.

FIGURE 31. - -U pperm ost rhyolite flow of the moat sequence overlying fine tuffs. View looking north from Dawings Pass. 88

10 CM

FIGURE 32. - - Hand specimens showing lamination in uppermost rhyolite flows in the moat. Samples are from Pine and Pinery Canyons. and biotite are present, and (3) magnetite is much larger. These differences in petrography suggest different sources for the rhyolite flow s.

Limited chemical data was obtained on the upper rhyolite

flows by X-ray fluorescence (Table 10). Because of the high

analytical error in the analyses, a sample from member 6 in the

Monument was analyzed as a control sample and the results on it

were compared with Enlows 1 published analysis. The data indicate

a close similarity between the moat rhyolite flow and member 6.

This suggests a close genetic association and consequently supports

the interpretation that the Turkey Creek caldera is indeed the source

for the tuffs since the moat rhyolite flows apparently originated within 89

TABLE 10. --Chemical Analyses on Rhyolite Canyon Formation and Moat Rhyolites (Weight Percent)

R h y o lite R h y o lite M o at Canyon Fm. Canyon Fm. R h y o lites m e m b e r 6 m e m b e r 6 (1) (2) (3)

S iC 2 7 6 .3 8 75.56 +1.40 76. 66 +1.40 a 12 ° 3 1 2 .4 7 13. 24 +1.20 14.20 +1.20 F e 2 0 3 1 .5 6 1 .1 0 + 0 .5 6 0 .8 5 + 0 .5 6 F e O T r . NA NA M gO . 15 .1 7 + 0 .1 3 .1 2 + 0 .1 3 C aO . 15 . 58 +0. 17 .1 5 + 0 .1 7 N a2C 3. 04 4 .7 0 + 0 .7 2 4. 92 +0. 72 k 2 o 4 .8 5 4. 96 +0. 01 5. 12 +0. 01 h 2 o 1. 11 NA NA T iO z . 12 NA NA P 2 O 5 . 01 NA NA M nO .0 5 NA NA 99.86 100.31 102. 02

NA= not analyzed for

(1) Near base of member 6; Enlows, 1955, p. 1233, sample 1. (2) 75 feet above base of member 6; field sample 592; X-ray fluorescence analysis. (3) Uppermost flow; field sample 628; X-ray fluorescence a n a ly s is .

See Appendix IV for X-ray fluorescence analytical techniques and sample locations. All Fe from X-ray analyses calculated a s F e 2 0 3 . 90 the caldera. In addition, it is interesting to note that the moat rhyolite appears to be even more differentiated than member 6, a s indicated by higher SiOg, AI2O3 and alkalis, and lower Ca, Fe, and

M g.

Monzonite Porphyry

The monzonite porphyry is a fine grained biotite-hornblende monzonite with an appreciable percentage of medium to coarse andesine phenocrysts. It is found forming the floor of the central cirque and steep slopes in the cirque wall rising hundreds to thousands of feet to meet the Rhyolite Canyon tuff in steep contacts. The monzonite is also found in the moat, where it forms annular segments a couple miles wide and totalling more than 120"of arc. Since the textural characteristics and stratigraphic relations of the monzonite

are significantly different in the cirque (or dome) and moat, the two

are discussed separately. The former will be referred to as the

dome monzonite, the latter as the moat monzonite. Andesites of

relatively small volume occurring in the moat were mapped with the

monzonite and they will also be described separately.

Dome Monzonite

Characteristics, The dome monzonite exhibits characteris­

tics which depend on depth of exposure in the pluton. These 91 characteristics appear to be due to alteration by vapors and fluids during crystallization of the pluton.

Outcrops of the dome monzonite are readily distinguished from Rhyolite Canyon tuff at a distance by the reddish or brownish color and hummocky appearance (Figure 33). The color can be extremely variable, especially at different depths of erosion and near contacts. It may include combinations of light brown, red- brown, grayish red and light gray. Fresh surfaces are commonly grayish olive with invading rusty browns. Orthogonal joints, spaced

6 to 12 inches, are common. Transverse joints are sometimes p r e s e n t.

The least altered samples (Figure 34) are found in deep canyons in the eastern part of the cirque, especially in Saulsbury and

Morse Canyons where depths of 1000 to 1500 feet in the pluton

are reached.

Petrography*- Modal data is given in Table 11. Phenocrysts

of rounded and zoned andesine (An 42), 2 to 4 mm in size, and

occasionally as large as 8"mm, constitute 10 to 15 percent of the

rock. They are best seen on slightly weathered surfaces (see Figure

39). The groundmass is holocrystalline, composed of 0.4 to 0. 8 mm

crystals of subhedral alkali feldspar, zoned plagioclase, euhedral 92

FIGURE 33. - -Typical outcrop of monzonite porphyry in the floor of the central erosional cirque. Note granitic-weather­ ing. Color is reddish brown.

FIGURE 34. - -Relatively fresh hand sample of monzonite porphyry from the dome. Location: along road cut in Morse Canyon 2000 ft. east of SE cor N E1/4, N E 1/4, sec. 24, T. 18S. , R. 29E. 93

TABLE 11. --Mode of Monzonite Porphyry-

Sample 631; 1500 counts; location given in Appendix IV.

Volume percent

Sanidine/Anorthoclase (2V<45) 44. 0 Plagioclase (An 42) 3 0 .4

Q u a rtz 1 0 .2 • B io tite 5 .4

H o rn b le n d e 4. 9 Pyroxene (diopside?) 2. 7

M a g n e tite 2 .4 100. 0

pyroxene, hornblende after diopside(?), and on a sm aller scale, rec­ tangular magnetite, interstitial quartz, and accessory apatite

(Figure 35).

Classification. Chemical composition of the rock was com­ puted from the mode and determined directly by X-ray fluorescence analysis (Table 12). Norms were calculated for both cases. The mode and norms are plotted in two different classification systems in

Figure 36. Modes are plotted in the system of Nockolds (1954), and norms, in a system derived empirically from Nockolds1 average norms by Jones, Hernon, and Pratt (1961). 94

FIGURE 35. - - Photom icro­ graphs of monzonite from the dome. Fresh sample; field number 631. Nicols; upper, uncrossed; lower, crossed. X40. 95 TABLE 12. --Chemical Analyses and Norms of Monzonite Porphyry Compared with Average Monzonite

M o n zo n ite M o n zo n ite Ave. hornblende- (petrographic) (X -ra y ) biotite monzonite (1) (2) (3)

S i0 2 60. 76 60. 97 +1.40 5 8 .4 5 A120 3 17.42 17. 75 +1.20 1 5 .6 5 F ® 2 ^ 3 4. 07 3 .2 1 + 0 .3 5 3. 34 F eO 2 .7 9 2. 17 +0. 21 3 .9 9 M gO 2. 54 2.40 +0. 87 2. 51 CaO 3. 73 3.44 +0. 26 5 .5 5 N a20 4. 10 5. 09 +0. 72 3 .4 7 k 2 o 4 .4 7 4 ,4 7 + 0 .0 1 4 .6 1 h 2o 0. 08 NA 0 .4 3 T iQ 2 NA NA 1 .4 0 M nO NANA 0. 09 P 2 ° 5 NA NA 0 .5 1 9 9 .9 6 99. 74 1 0 0 .0 0

O r 2 6 .6 9 2 6 .6 9 27. 2 Ab 34. 58 42. 97 29. 3 An 1 5 .8 5 1 2 .2 3 1 3 .3 Mg 5 .8 0 4 . 64 4 .9 11 2. 7 Ap 1 .2 Di 2. 22 3. 73 Hy 7. 18 5. 56 C aSiO s 4. 5 M gSiOg 6. 3 FeSiO g 2 .4 Q 7. 62 3 .6 6 7. 7 9 9 .9 4 9 9 .4 8 9 9 .5

NA= not analyzed for (1) From mode except KgO which is from flame photometry. K/Na ratio of feldspar taken as that which gives this KgO. Sample 631; sample location given in Appendix IV. (2) KgO by flame photometry. Fe computed as FeO + FegOg to the same molecular ratio as in the petrographic analysis. Sample 551. Analytical and sample information is in Appendix IV. (3) Average of 5 analyses, Nockolds, 1954, p. 1017. 96

q u a rtz

q u a rtz d io r ite A. M ode quartz \grano- g ra n ite monzonite \ diorite d io rite t s e n ite m o n z o n itesy

a lk a li feldspar (An<50)

quartz monzonite granodiorite q u a rtz

q u a r tz B . N o rm d io r ite

g ra n ite

sy e n ite d io r ite

syenodiorite m o n z o n ite

1 Calculated from petrographic mode. 2 Calculated from X-ray fluorescence chemical a n a ly s e s .

FIGURE 36. --Mode aqd norms of monzonite porphyry plotted in two classification systems. Limiting proportions in modal plot, A, are those of Nockolds (1954) except that 33 and 66 percent limits are used as intercepts at the base of the triangle instead of 40 and 60. respectively. Limiting proportions in normative plot, B. were derived empirically (Jones, Her non, and Pratt, 1961, p. 12) from a plot of Nockolds' average norms. 97

In the modal classification, this rock falls just above the

10 percent quartz line and would be classified as a quartz monzonite.

In the normal classification, the petrographic norm plots in the monzonite field while the X-ray norm plots in the syenodiorite field.

When we correct for the fact that the X-ray chemical analysis of

NagO is as much as 1.7 percent high, & this point is shifted to the

left and well within the monzonite field.

In view of the hypabyssal nature of this rock, the normal

rather than modal classification should be used, in which case it is

classified as a monzonite. The chemistry also compares well with

Nockolds1 average hornblende-biotite monzonite (Table 12).

Chemically, the rock is not akin to a syenodiorite, since

the latter contains more Ca, Mg, and Fe. In addition, the

petrography is more characteristic of a monzonite than a quartz

monzonite considering the absence of alkali feldspar phenocrysts

and the abundance of pyroxene and mafic m inerals. At depth the

pluton may be even less silicic, because of assim ilation of rhyolites

near the roof.

Age and Correlation. The monzonite porphry is post-

Rhyolite Canyon, since it intrudes Rhyolite Canyon tuff, but is very

6. Comparison of X-ray analysis of member 6 with published analysis in Table 10. 98 close in age as indicated by K-Ar ages on a dike intruding it

(24. 8 +0. 7 m. y. , DM-3-68, Appendix II) and on the Rhyolite

Canyon tuff (the mean of two dates obtained on the tuff is 24. 9 +0. 6 m . y . ).

Incidentally, it is noted that the monzonite porphyry is sim ilar in age, petrography, and chemistry to the rhyodacite flow which caps the Rhyolite Canyon ash-flow sequence in the Monument

(member 9, Figure 8). The rhyodacite contains large plagioclaee phenocrysts that are about the same size, composition, and abundance as in the monzonite. There is less hornblende and biotite in the rhyodacite however. The norms of the rhyodacite (Enlows, 1955, p.

1235) and monzonite are close but quartz is 10 percent higher in the rhyodacite. The overall sim ilarity of these rocks suggests a cogenetic relation.

Nature of Contacts. This discussion supplements a previous discussion in this chapter on the characteristics of welded tuff near an intrusive contact.

Intrusive contacts along the wall of the central cirque are

steep, sharp, and near linear. This suggests that they were fault- controlled. It looks like the monzonite was injected between large fault blocks, on the order of a mile across, which were uplifted by different amounts. There is no evidence for assim ilation and the 99 contacts are quite sharp. In one contact which was examined, ? there is no evidence of brecciation or incorporation of the welded tuff and the welded tuff can be distinguished from the monzonite except for a zone less than an inch wide.

Near the contact the monzonite is highly fractured (Figure

37). As the contact is approached, the fractures trend alternately parallel or perpendicular to the contact suggesting stress or flow patterns. At the contact itself the monzonite shows no unusual characteristics except perhaps slightly finer texture.

A contact with the "central pendant" of welded tuff®(northeast corner, sector IV) is much less steep, dipping about 45® under the pendant. It is also more complex, with a 20-foot wide brecciated zone and a 50-foot wide extensively altered zone. In the latter, prim ary textures are completely obliterated.

Moat Monzonite

The moat monzonite is found as three separate segments about 2 miles wide, lying against or in some cases possibly over the caldera wall. Together they extend nearly 150® of arc in the moat.

7. South of Cottonwood Canyon, SW 1/4, sec. 28, T18S, R 29E .

8. West of Polebridge Canyon in NE 1/4 and SE 1/4, sec. 24, T18S, R29E. 100

The moat monzonite forms pale light gray cliffs several hundred feet high on the west side of Buena Vista Peak and on the south side of Barfoot Peak (sector II). About 3000 feet has been exposed in a magnificent scarp along the eastern wall of the caldera

(Figure 38).

General Description. The monzonite in the moat is generally more finely crystalline than in the dome, probably due to chilling near contacts and less overburden. In a typical sample, the m atrix consists of crystals in the 0. 1 to 0. 3 mm size range. A sample from middle

Rucker Canyon, however, showed the same degree of crystallization as in the dome.

In the moat the monzonite seems to have a higher percentage of phenocrysts than in the dome, as high as 30 percent (Figure 39).

They are in the same size range but are more calcic, near An59.

Magnetite is less abundant, hornblende more abundant, and secondary hematite is present.

Characteristics of the moat monzonite in sector II indicate that the top had solidified at or very near to the surface. In places, the upper portion is chilled to a dark gray glassy rock. In the vicinity of San Mateo Point, one mile northeast of Flys Peak, spectacular flow structure is observed (Figure 40). This zone is about 25 feet thick and overlies normal crystalline monzonite. Centimeter-size vesicles 101

FIGURE 37. --F ra c u rin g of monzonite near steep contact in the dome. For scale, slabs on the left are 6 in. wide.

FIGURE 38. - - View of escarpment forming eastern side of caldera. Erosion has removed thick section of rocks forming the caldera wall, cutting at present into Cretaceous limestones (A), and exposing some 3000 feet of monzonite in the moat (B). Winn fault is immediately this side of the lower cliff. Note appearance of zoning in the monzonite section and characteristic southward dip. View looking west from the ridge just northeast of Winn falls. 102

FIGURE 39. - - Hand specimen of monzonite from the moat. Secondary alteration has resulted in the prominent light colored andesine ghosts. Sample is from the vicinity of Rustler Park.

FIGURE 40. - -Spectacular flow structure in the uppermost part of the moat monzonite. Location: one mile northeast of Flys Peak at San Mateo Point. 103 lined with quartz crystals amount to 20 percent of the rock. Quartz xenocrysts are also present. The presence of quartz xenocrysts is anomalous in the monzonite and suggests incorporation of silicic ro c k s .

Stratigraphic Relations and Age. In the few locations where contacts were encountered (along the caldera wall, in sectors II and

IV) the moat monzonite overlies older rocks forming the wall. The contact dips 20° to 45“ inward to the caldera.

A most informative contact was encountered near Winn Falls, directly beneath the cliffs that form the scarp along the eastern wall of the caldera9 (Figure 38). The dark gray, aphanitic (chilled) base of the moat monzonite, dips 20° inward to the caldera, overlying a sequence of rhyolite tuffaceous sediments and bedded tuffs (Figure 41).

Only about 30 feet is exposed below the contact. These units were examined for fragments of welded tuff but none were found. Blocks consisted of rhyolite flow, macroscopically and m icroscopically sim ilar to the youngest flows of the moat rhyolites.

Although a full interpretation of this contact would require further study, at least some deductions can be made. First, the

9. Contact is exposed on the north side of the canyon leading up to Winn Falls. 104

FIGURE 41. --Tuffaceous rocks underlying monzonite near the caldera wall in the moat. Blocks and lapilli are rhyolite flow. Location: base of cliffs on north side of canyon lead­ ing to Winn Falls.

blocks in these sediments were most likely rhyolites extruded in the caldera. The sediments were probably deposited in the caldera floor or moat. Secondly, the monzonite was emplaced over the

sediments, apparently butting up against a fault scarp defining an

outer ring-fault of the caldera. The fault is only tens of feet east

from the contact. Relations at this contact will be referred to

later in the discussion.

A very informative section of the moat rhyolites is exposed

below Ida Peak (sector II; Figure 42). The Rhyolite Canyon tuff is 105

Map Rock Description u n it* ______ty p e______

moat rhyolite Total thick, about 800 ft. rhyolites flow Brecciated base.

About 100 ft. thick. Vesicular Z *+* Ida P e a k and brecciated at the top. ' + + 4- a n d e s ite monzonite tuff Thin bedded with sand­ porphyry breccia & stone lenses, lapilli and s e d im e n ts blocks**. Brecciatedtop^ •i m o n z o n ite Intrusive contact at p o rp h y ry base; basal breccia. Crystal-rich, upper(?) Rhyolite rhyolite mem. Crystallized Canyon welded near upper contact. F m . tu ff

P la te 1

Rounded to subrounded, composed of 95% crystal-rich, upper(? ) Rhyolite Canyon tuff.

FIGURE 42. —Schematic north-south section through Ida Peak on its south side. Total section about 800 ft. 106 brecciated, remobilized and crystallized at the contact with the moat monzonite (Figure 24). Quartz dikelets also intrude the tuff.

Alteration continues for hundreds of feet from the contact. Relations at the upper contact would be valuable in establishing the age of the moat monzonite but the contact was not found exposed. It is expected

that the overlying tuffaceous rocks are in depositional contact because

they are not noticeably altered.

It was noted earlier that an erosional unconformity occurs

immediately above the lower rhyolite flows (Pine Canyon; location D,

Figure 26) and in sedim entary units in Rucker Canyon (Figure 27).

This unconformity divides the moat rhyolites into two groups: these

are referred to here only as the upper and lower groups. The lower

group generally consists of flows and overlying sediments. The flows

are those found in the bottom of Rattlesnake Canyon, in Pine Canyon,

and upper Rucker Canyon (Figure 26 and Table 9). They are es­

pecially thick in Rucker Canyon. These flows are generally distinct

from younger flows by their darker colors and greater abundance of

mafic m inerals. At present the lower group is not in sectors II and

IV, where the moat monzonite is present.

All rocks of the upper group, however, including tuff

breccia and sediments, fine tuffs, and flows, are found throughout

the moat but in greater or lesser thickness depending upon whether 107 they overlie moat monzonite or not. For example, these rocks are observed to thin eastward toward the Fife fault beyond which is the moat monzonite. In sector I they approach 1000 feet in thickness yet below Ida Peak the sediments and tuffs are much thinner. Another example is along the ridge of the range. In Rucker Canyon, the upper group exceeds about 1200 feet in thickness (Table 9). To the north of this on the ridge, all the units are present but thinner by a factor of ten or more. The sum total of this discussion is that we have an age on the emplacement of the moat monzonite as post-

1'lower group" or following deposition of part of the moat rhyolite sequence. That is, it was emplaced, in sector II at least, following a period of extrusion of aphanitic rhyolite flows,sedimentation in the caldera or moat, and prior to the emplacement of the largest part of moat deposits presently exposed, including: tuffaceous sediments and tuff breccia, fine tuffs, and the uppermost flow.

A n d e s ite s

Porphyritic andesite flows are found above the moat monzonite at Ida Peak and near Long Park (sector II). Andesites in these areas are discussed separately in view of slightly different

stratigraphic relations and petrography.

The Ida Peak andesite forms prominent 100-foot cliffs on

Ida Peak seen from the south or west (Figure 42). It is 108 stratigraphically separated from the monzonite porphyry by tuffaceous sediments and breccia. Its time of emplacement is after the tuff breccia subunit of the moat rhyolites but prior to the upper rhyolite flow which caps the Ida Peak section.

This rock is medium dark gray with 10 to 15 percent rounded andesine (An46) phenocrysts, typically 1 to 2 mm but as large as 4 to 10 mm in size. Magnetite is abundant. Some flattened vesicles containing quartz crystals may be seen. Again, these could be due to the incorporation of rhyolite. This rock differs from the moat monzonite in having an aphanitic groundmass and half as much phenocrysts by volume.

The Long Park andesite is exposed on the road cut northeast

and east of Long Park, and in the canyon between Long Park and

Flys Peak. In the latter, it appears to grade downward into the

monzonite porphyry as if it were an extrusive phase. In the form er

it underlies the tuff breccia subunit of the moat rhyolites. Its time of

extrusion is therefore between the time of emplacement of the moat

monzonite and the tuff breccia subunit of the moat rhyolites. This

andesite is more reddish than the Ida Peak andesite. The percentage

of phenocrysts is about the same. Plagioclase is characteristically

zoned and occurs in complex groups (Figure 43). It is also more

sodic (around An33) than either the Ida Peak andesite or monzonite 109

FIGURE 43. - -Photomicrographs of Long Park andesite. Showing intergrown andesine phenocrysts. Nicols: left, uncrossed; right, crossed. Upper pair: X40. Lower pair: XI5. n o porphyry. Hematite is associated with magnetite. Clinopyroxene is after plagioclase. Overall mineralogy suggests that it is not cogenetic with the Ida Peak andesite.

It is noted that the Long Park and Ida Peak andesites are classified as andesites only on the basis of phenocryst composition.

Their overall characteristics and age suggest a genetic relation with the monzonite and the rhyodacite of the Rhyolite Canyon Form ation

(member 9).

D ik es

Only five dikes were mapped in the caldera in the course of this investigation (Plate 1). Many more may exist. These dikes

are small, only 10 to 100 feet wide and exposed for less than a mile

along their strike.

Four of these dikes are in the moat where they intrude rocks

tentatively correlative with Rhyolite Canyon tuff but whose positive

correlation is obscurred by alteration and crystallization. The dike

rocks are generally black glassy and perlitic. They are usually

porphyritic with medium sized phenocrysts. Biotite may be present.

One dike intrudes the monzonite porphyry in the dome.

This is a biotite rhyolite dike with a cryptooxystalline m atrix. A

K-Ar age of 24. 8 +0. 7 m. y. (DM-3-68) obtained on this dike was

crucial in the identification of the caldera because it showed that I ll the moat monzonite is close' in age to the Rhyolite Canyon

tuff.

The only knowledge of the age of these dikes is that they

are probably post-Rhyolite Canyon. It is very likely that they are

related to resurgence.

Epis (1956) mapped three north-trending dikes in the vicinity

of Rucker Canyon, just south of Rucker fault which lies along the

southern wall of the caldera (Plate 1). Nothing is certain regarding

their age except that they intrude Cretaceous rocks. They could be

time -correlative with dikes in the caldera. If so, this would be

evidence that the Rucker horst block was uplifted at the same time

as resurgence of the caldera. C H A P T E R VI

COMPARATIVE PETROGRAPHY, CHEMISTRY AND K-Ar AGE

OF ROCKS ASSOCIATED WITH THE CALDERA

Homogeneity

More than a dozen separate rhyolite volcanic units are recog­ nized in the sequence of rocks related to the Turkey Creek caldera. i Petrographically and chemically, these rhyolites are remarkably

homogeneous. Phenocrysts are composed of quartz, sanidine and

magnetite, in close relative proportions by volume percent. The two

oldest rhyolites in the sequence, members 1 and 2 of the Rhyolite

Canyon Formation, are notable exceptions. Member 1 contains an

occasional hornblende crystal (Enlows, 1955, p. 1221) and member

2 , a small percentage of biotite (Table 2). The overall chemical

composition of the youngest rhyolite, the uppermost flow in the moat

rhyolites, agrees well with that of member 6 (T ab le 10). K , R b,

and Sr data on the rhyolites are closely grouped (Table 13; Figures

44 and 45). K ranges from 4. 00 to 4. 36 percent, Rb from 249 to

436 ppm, and Sr from 3. 5 to 19. 2 ppm. Finally, K-Ar dates obtained

on three units in the sequence (Table 14) indicate a relatively short

tim e-interval from the initial ash flows to caldera resurgence

112 113

TABLE 13. --K, Rb, and Sr Analyses of Caldera Igneous Units

K analyses by flame photometry (Appendix II); Rb and Sr by X-ray fluorescence (Appendix IV). Sample information is in Appendices II and IV.

Field Member K Rb Sr K/Rb Rb/Sr no. no. % ppm ppm Whole rock- moat rhyolites, uppermost flow

568 - 4.19 +.01 395.4 +9.5 5.7 +0.6 106.0 +2.5 69.4 +7.5 628 - 4.248 +.004 403.9 +4.8 3. 5 +0.6 105.2 +1.2 115 +20

Whole rock- porphyritic rhyolites

322 - 4.36 +.01 330.0 ±4.0 6. 2 +0. 6 132.1 +1.6 53.2 +5.2

Whole rock- Rhyolite Canyon Formation, Chiricahua Peak

400 - 4.22 +. 02 248. 7 +1.8 16.7 +0.6 169.7 +1.4 14.89 +0.55

Whole rock- Rhyolite Canyon Formation, Monument

595 8 4.35 +.02 302.3 +2.0 16.2 +0.6 143.9 +1.1 18.66 +0.70 596 6, top 4.051 +.007 377.3 +4.5 19.2 +0.6 107.3 +1.3 19.65 +0.66 592 6, bottom 4.119 +.007 416.5 +2.6 6. 7 +0. 6 98. 92 +0. 54 62.2 +5.6 613 4, top 4.034 _+. 004 435.5 +2.8 14. 2 +0. 6 92.54 +0.58 30.7 +1.3 597 4, bottom 4.009 +.008 433.2 +1.3 6.8 +0.6 92.57 +0.36 63.7 +5.6 602 2, top 4.05 +.01 301.7 +3.6 11.5 +0.6 134.2 +1.6 26.2 +1.4 614 2, bottom 4.056 +.008 324.8 +3.9 14.5 +0.6 125.0 +1.5 22.40 +0.96

Whole rock- monzonite porphyry

551 - 3.71 +.01 165.3 +1.8 266.7 +3.6 224.4 +2.5 0.620+0.011 631 - 3.939 +.007 174.5 +1.4 253.5 +2.0 225.8 +1.9 0.688+0.008

Whole rock- lower rhyolites, Faraway Ranch Formation, Monument* 611 7 4.01 +.02 176.1 +1.3 224.5 +2.0 227. 7 +2. 0 0.784+0.008

Sanidine- Rhyolite Canyon Formation, Monument** 69 8 6.315 +.030 138.6 +1.5 14.9 +0.6 455.6 +5.2 9.30 +0.39 71 2 6.75 +.04 141.3 +1.5 7. 3 +0. 6 477.7 +5.8 19.4 +1.6

*This unit is not considered part of the caldera sequence. Data included for reference.

**Splits of mineral separates used for K-Ar analysis (Appendix II). 114

10 5

KEY

11 monzonite porphyry 10 moat rhyolites, uppermost flow 9 porphyritic rhyolites 8 Rhyolite Can. Fm. , top of Chiricahua Peak 7 f 'm e m b e r 8

5 Rhyolite m e m b e r 6 , b o tto m 4 Can. Fm. — 3 M o n u m en t member 4, bottom

m e m b e r 2 , b o tto m

Rb p p m

FIGURE 44 . - -Plots of K vs. Rb for caldera sequence. Dashed line diagonals indicate range for "normal11 igneous rocks, after Taylor (1965, p. 142). 115

1000

l I I I I I

S r p p m

FIGURE 45. - -Plots of Rb vs. Sr for igneous rocks associated with the caldera. Key same as Figure 44 .

TABLE 14. --K-Ar Ages of Caldera Units

Unit Laboratory K -A r ag e no. (m . y. )

Dike intruding monzonite D!M—3-6 8 24. 8 +0. 7

Rhyolite Canyon Fm. , member 8 D M — 3 —67 24. 9 ±0. 7

Rhyolite Canyon Fm. , member 2 D M -2 -6 7 25. 0 +0. 8 116

(emplacement of the monzonite), much less than the predicted mean standard deviation of a single analysis, 730,000 years.

The close petrologic and chemical characteristics of the rhyolites, along with their close association in space and time, indi­ cate that they were derived from a common parent magma. Since at least some of these rhyolites are mainly confined to and undoubtedly originated within the caldera, this is evidence that the Turkey Creek caldera is indeed the source area of the Rhyolite Canyon tuffs.

Vertical Variations

The ash-flow sequence exhibits vertical mineralogical and chemical variations sim ilar to those described in other ash-flow sequences (Lipman, Christiapsen, and O'Connor, 1966; Smith and

N Bailey, 1966; RattS and Steven, 1967; Ewart, 1965; and others).

Phenocryst abundance and sizes generally increase upward in the sequence, going from 5 to 15 percent abundance (2mm maximum size) near the base to 20 to 35 percent (4 mm) at the top (Table 2).

Similarly, the size-distribution of quartz phenocrysts broadens up­ ward in the sequence as the modal size increases from 3/4 to 1 m m

near the base to 1-1/2 to 1-3/4 mm at the top (Figure 51, Appendix

HI). If the inverse relation between total phenocryst percentage and

SiOg percentage holds for this sheet as it commonly does in ash-flow 117 sheets (see for example, Lipman, Christiansen, and O'Connor,

1966, p. 8 ), this means a decrease in SiO^ content upward.

The relative percentage of phenocrysts shows a trend as well, with sanidine content going from about 56 percent in member 2 to 66 percent in member 8 and magnetite from about 2 to 4 percent in the same manner (Figure 50).

K and Rb concentrations show trends parallelling petro­ graphic variations. The younger rhyolites tend to have higher K, around 4. 3 percent, compared with around 4. 0 percent for the oldest ash flows (Figure 44). Rb decreases upward from the base to the top of member 2 and from member 3 to the Chiricahua Peak unit. The decrease of Rb upward in an ash-flow sequence was also noted by

Smith and Bailey (1966, p. 19). The discontinuous change in Rb across the unconformity between members 2 and 3 implies some sort of chemical or petrologic readustment during the intervening hiatus.

If so, the amount of Rb discontinuity might be used as an indication of the magnitude of such an adjustment or perhaps even the magni­ tude of the hiatus.

In regard to changes in Rb concentration, one must consider

the affect of phenocryst content. Since Rb is concentrated in the melt,

an increasing phenocryst content generally means lower Rb. Actually,

some quick calculations show that the spread in Rb concentration in 118 the rhyolites indicated in Figure 44 would be reduced to about half if they all had the same phenocryst content. The Rb content of sani- dine in two of the ash flows is given in Table 13.

No trend in Sr content is indicated by the data (Figure 45).

This is not surprising since there is no mineralogic system present in the rock (such as calcium-bearing crystals) which would fractionate

S r. CHAPTER VII

STRUCTURE OF THE CHIRICAHUA MOUNTAINS

In the course of unravelling the stratigraphy in the Chiri- cahua Mountains, a basic understanding of the structure evolved.

Some mention of structure in isolated areas has already been made in Chapters IV and V. In the present chapter major structural features of the range and of the caldera are described.

Major Structural Features

In the Chiricahua National Monument several periods of structural adjustment are recorded in the m id-Tertiary sequence, ranging in age from 28. 9+1.9 m. y. (DM-2-68) to 24. 9 +0. 7 m. y.

(DM-3-67). Elsewhere in the range two main periods of tectonic activity are recognized by their effect on the ash-flow sheets. The first of these periods immediately followed eruption of the middle of three major ash-flow sheets and the second followed deposition of the upper or Rhyolite Canyon sheet.

The age of the first tectonic period is bracketed by K-Ar ages on the middle and upper ash-flow sheets. Two dates from near the top of the middle sheet are: 25. 7 +0. 8 m. y. (DM-6-67)

on the Cave Creek formation and 24. 5 +_1. 2 m .y. (DM-1-68) on

119 120 lower rhyolites in Horseshoe Gmyon. Age of the Rhyolite Canyon sheet is 24. 9 +0. 6 m . y.

The net effect of tectonic activity in this period was block faulting, with parallel faults separated by about a mile, and westward tilting of 10° to 15* . If one is looking for broad doming of the range prior to eruption of the Rhyolite Canyon ash flows, there is a ten­ dency for known faults in the Cave Creek, Horseshoe Canyon, and

Shake Gulch areas to show concentric orientation. Horseshoe fault

(Plate 1), located about 2 miles outside the caldera wall, is a good candidate for such a fault, It was apparently activated before erup­ tion of the Rhyolite Canyon ash flows, since the ash flows are much thinner on the upthrown side. The Horseshoe fault was active until after caldera resurgence. Its total displacement must be several thousand feet.

Two major faults, the Cholla and Sunset faults, are radial to the caldera, bounding the 4-mile wide Rucker horst block. The

Sunset fault is accompanied by hard siliceous, hematitic flows that are interlayered with units correlative with the moat rhyolites.

Displacement on this fault is estimated at several thousands of feet. Sim ilarities with the Horseshoe fault suggest that it may be of the same age. 121

The Rucker horst block extends northward through the caldera ring-fracture zone. Fife and Dawings faults in the northern part of the caldera may be northward extensions of the Cholla and

Sunset faults, respectively.

The Winn fault is a.major caldera ring fault. Although studied in only a few locations, the stratigraphic discontinuity re­ quires that it extend more than 70° of arc. Aside from this one, ring faults are generally obscured by: (1) alluvial deposits; as in the western valley and most of sector I, (2 ) moat rhyolites which overflowed the walls, as in sector III, and (3) moat monzonite, which appears to have been emplaced in such a way in sectors III and IV so as to conceal the presence of ring faults, i. e. , by being

injected into or over the caldera wall rocks.

At one point in this study, middle Rucker Canyon was

thought to be an outer ring fault on the basis of the following obser­

v a tio n s : (1) the south wall of the canyon is characterized by alter­

ation and hematitic flows sim ilar to those accompanying major

faults like the Sunset fault, knd (2) the canyon is located about

where the outer ring-fractures are expected. No evidence of a

major stratigraphic discontinuity in the canyon was found, however, : ' i'.- and also there is no major discontinuity south of the canyon. This

would imply that the major caldera faults are north of the canyon. 122

The doming of Rhyolite Canyon tuffs in the caldera is pronounced. Dips are radial and 20*- 35* nearest the center of the

dome (in the walls of the central erosional cirque) and generally

sm aller, around 15° , in the moat region. Rhyolite Canyon tuff is

exposed in the moat in the southern part of the caldera, sector IV.

Some blocks in the western most exposures near the valley are

subvertical and appreciably altered.

Several major sub-radial faults were mapped in the moat.

Of these the Fife fault and a fault paralleling it, just to the east, are

dated by their affect on the moat deposits. These faults appear to

post-date the moat sediments but pre-date the uppermost rhyolite

flows. If faults such as these formed as a consequence of doming,

this means doming continued throughout the sedim entary phase but

had virtually terminated by the time the latest lavas erupted.

No attempt can be made at this stage to correlate post-

Rhyolite Canyon activity in the caldera with that outside the caldera.

In the latter, Rhyolite Canyon tuffs were cut by parallel faults,

spaced on the order of a mile, with generally small vertical dis­

placements within the range. In addition, westward tilting of 10®

was imposed.

The major block faulting which separated the Chiricahua

range from the adjacent San Simon basin must have taken place 123 after deposition of the Rhyolite Canyon sheet. This is indicated by

the presence of Rhyolite Canyon tuff in the Peloncillo Mountains.

If the basin-range structure was formed prior to emplacement of the

Rhyolite Canyon sheet, filling of the intervening valley with a thickness

of tuff in excess of a mile would have had to have preceded deposition

on the Peloncillo range. In addition, extensive erosion of this fill

would have been necessary to produce the present topography.

Since this evidence on the age of formation of basin-range

structure is so important, the correlation on which it is based should

be checked. K-Ar and chemical data should be obtained to verify the

correlation of Rhyolite Canyon in the vicinity of South Antelope Pass

in the Peloncillo Mountains.

Amount of Caldera Subsidence

Estimates of caldera subsidence can be made by determining

the magnitude of the stratigraphic discontinuity at the wall. Discon­

tinuities can be seen in cross-sections through the wall in Plate 1.

Consider first the section through Pinery Canyon (section

A-D, Plate 1), idealized in Figure 46. The discontinuity, d, is

d = a + b + c

where the base of the Rhyolite Canyon sheet is taken as a reference.

The quantity "c", measured from Plate 1, is 600 feet. The quantity 124

"b" is the thickness of moat deposits not yet South N o rth ring-fracture exposed at this location. m o a t w a ll zo n e The section of moat deposits that is exposed includes the fine tuffs and uppermost flow but not the tuff breccia and sediments. The maxi­ FIGURE 46. - -Idealized section through Pinery Canyon. See Plate 1 mum thickness of this for explanation of symbols. part of the moat sequence is about 300 feet (Figure

26) and this may be taken as a minimum estimate for "b". The quantity "a", the thickness of Rhyolite Canyon tuff in the caldera, is about 3000 feet (Table 6 ). With these numbers we get a discontinuity

d = 3000 + 300 + 60 0 0 , or 3900 feet.

The same type of calculation can be made for the section through Chiricahua and Sentinel Peaks (section I-J, Plate 1), idealized in Figure 47. Here the discontinuity is

d = a + b - c .

The quantity "c", measured from the cross-section in Plate 1 is 125 about 800 feet. The Chiricahua Sentinel total thickness of moat P e a k P e a k ring-fracture rhyolites, "b", may be moat zone wall taken as 1400 feet T m r T m r (T ab le 8 ). Thickness of Rhyolite Canyon tuff, T r l & pT "a" , is again 3000 feet.

These numbers give a discontinuity of FIGURE 47. - -Idealized section through Chiricahua and Sentinel d = 3000 + 1400 - 800 Peaks. See Plate 1 for explanation of symbols. or 3600 feet.

The largest factor in these calculations is the thickness of Rhyolite Canyon tuff in the caldera, particularly right at the wall. The numbers used above were from exposed sections some three or four miles inside the wall.

It is noted that these calculations give the time - integrated subsidence, i. e. , the total amount the caldera floor subsided during its history. This may have been a case in which the caldera was being filled with volcanic m aterial as fast as it was subsiding so that the actual topographic discontinuity at the wall at any one time 126 would not have been great. Such was the case with the Creede

caldera (Steven and Ratte, 1965, p. 14).

The value of 4000 feet obtained is reasonable for a caldera

of this size. Cornwall (1962, p. 370) measured the subsidence along

a ring fault as 3500 feet in a caldera of around the same diam eter. CHAPTER VIII

UNUSUAL FEATURES OF THE TURKEY CREEK CALDERA

As stated earlier, the Turkey Creek caldera closely resem ­ bles other calderas of the Valles type in ( 1) overall dimensions and s h a p e , (2 ) characteristics of the rocks present in and around the caldera: rock types, distribution, relative abundance, volume, order

of emplacement, time-span, petrologic and chemical relations, and

alteration, and (3) structures: major stratigraphic discontinuities,

ring faults, and central dome.

The caldera differs from known Valles type calderas,

however, in its surface expression and in some structural and

stratigraphic features.

To get a feeling for what erosion has done to the Turkey

Creek caldera since its formation, we may take the relatively

uneroded Valles caldera (of Pleistocene age) as a model of what it

looked like originally and compare it with the present surface

expression. In the Valles caldera, the walls are some 500 feet

above the top of rhyolite domes in the moat. In the east wall

of the Turkey Creek caldera, erosion has removed the wall

rocks to a depth of 3200 feet below the level of the uppermost

127 128 rhyolites in the moat. Allowing a conservative 1000 feet for moat rhyolites already removed, we find that the wall has been eroded to a depth of 4700 feet below its original level. This also means that rock units inside the caldera (in the moat) are exposed to a depth of

4200 below their original level in this scarp.

We can make the same type of computation for the dome.

In the Valles caldera the central dome rises 2000 feet above the

top of the rhyolite domes in the moat; in the Turkey Creek caldera

erosion has cut down to a level of 2400 feet below the highest level of

moat deposits. Again allowing 1000 feet for the thickness of moat

deposits already removed we find that erosion has exposed rocks

5400 feet below the original top of the central dome.

Exposure of the monzonite in the central dome and ring-

fracture zone is a consequence of this deep erosion and a unique

feature of the Turkey Creek caldera. What is the structure of this

monzonite body? Only a hypothesis can be put forth at this point. The

complete absence of pre-Rhyolite Canyon (and perhaps even lower

members of the sequence) in the caldera, coupled with the apparent

inability of the monzonite to assim ilate as a consequence of its low

emplacement tem perature, suggests that the monzonite forms a

laccolith (Figure 48). The monzonite may have been emplaced in the

thick, relatively incoherent tuff beds immediately underlying the Turkey Creek Caldera

Turkey C a v e feet Ch 12000-1 Trc Trl : / * ‘ • - * » . ; Trc & 8000*

4000- I Trc

4 M ile#

No vertical exaggeration

I Tmr I Trc | I TH | I PT | I T m p 1 moat rhyolites Rhyolite Can. lower rhyolites pre-Tertiary m onzonite F m . p o rp h y ry See Plate 1 for further explanation.

FIGURE 48. --Composite cross-section through the Turkey Creek caldera with hypothetical interpretation. 129 130

Rhyolite Canyon sheet. Smith (I960) suggested that a central

stock or laccolith was responsible for doming of the Valles caldera.

More recently, however, Smith and Bailey (1968, p. 645) indicated that laccolithic intrusion was not favored as a cause of resurgence.

The Turkey Creek caldera does not appear to have the

central graben which is characteristic of calderas of this type. Actu­

ally, a couple horst structures transect the caldera: the northward

extension of the Rucker horst block, bounded by the Cholla and Sunset

faults, and the block bounded on one side by the Fife fault. The central

"pendant" of tuff (northeast corner of sector IV) could have been part

of a graben structure, however.

The relative paucity of sediments in the moat and the absence

of lacustrine deposits appears to be an anomalous feature of this

caldera. The thickness of sediments and tuff breccia does not exceed

a few hundred feet, almost an order of magnitude sm aller than in the

Valles and Creede calderas. Furthermore, in sector II where they

overlie the moat monzonite, they are less than 50 feet thick. Two

explanations may be considered: ( 1) thick sequences of sediments

were never deposited in the moat because of a short time interval

between caldera collapse and filling of the moat with tuffs and flows,

and (2) thick sequences were deposited in sectors I and III but are not

yet exposed and deposition in sectors II and IV was inhibited by the 131 emplacement of the monzonite. The latter explanation is favored by the evidence mentioned in Chapter V that the moat monzonite was emplaced amidst deposition of sediments in the moat.

The absence of basaltic rocks appears to be another anomalous feature of the caldera and this volcanic field since the ande s ite - rhyolite - basalt sequence is common to caldera sequences and continental ero­ genic zones (Beloussov, 1962, pp. 649-650). The hypabyssal intrusion of monzonite in the dome and moat probably represents the basaltic phase. C H A P T E R IX

SUMMARY OF THE TERTIARY GEOLOGIC

HISTORY OF THE CHIRICAHUA AND

NORTHERN PEDREGOSA MOUNTAINS

The Tertiary history of the Chiricahua and Northern

Pedregosa Mountains is essentially that of rhyolitic ash-flow deposi­ tion and coneommitant block faulting and caldera formation in the period from 29 to 25 m. y. Rhyolites and lesser amounts of volcanic

sediments and andesitic rocks are locally interbedded with the ash-

flow deposits.

Rhyolite tuffs dated at 28. 9 +JL. 9 m. y. (DM-2-68, Appendix

II) form the base of the Tertiary sequence in the northern part of the

range. Following this, amid periods of structural adjustment,

basaltic andesite flows and breccia, alluvial-fan sediments, and

a great rhyodacite flow were deposited. The rhyodacite dated at

2 7 .6 +0. 8 m. y. (DM-1-67), covered some 50 to 100 square miles

in the northern Chiricahua Mountains where it was covered by the

2000-foot thick Rhyolite Canyon ash-flow sheet at 24. 9 +0. 6 m . y.

(average of DM-2-67 and DM-3-67).

132 133

In the eastern part of the range two m ajor ash-flow sheets, each around 1000 feet thick, preceded the Rhyolite Canyon sheet.

These are referred to as the lower and middle sheets. In one area thick rhyolite flows, more than 1000 feet thick, separate the two sheets. The middle sheet, dated at 24. 5+1.2 m. y. (DM-1-68) and

25. 7 +0. 8 (DM-6-67), is indistinguishable in age from the Rhyolite

Canyon sheet. It was affected by block faulting and westward tilting of 15* on the average prior to eruption of the Rhyolite Canyon sheet.

The source areas of the lower and middle sheets are probably in the eastern part of the range or in New Mexico,

Eruption of the Rhyolite Canyon ash flows from the western part of the range resulted in the formation of the 13-mile diam eter

Turkey Creek caldera. These ash flows spread over some 700 square miles, flowing more than 10 miles north of the caldera (where they are exposed in the Chiricahua National Monument), 20 m iles to the east (in the Peloncillo Mountains), 18 m iles to the south (in the northern Pedregosa Mountains), and 12 miles to the southwest (in the Swisshelm Mountains). Original volume of deposits is estimated to be of the order of 100 cubic m iles.

Resurgence of the caldera was associated with intrusion of

a monzonite porphyry. The monzonite is exposed in the central

dome and in the ring-fracture zone where it forms annular elongate 134 bodies, about 2 miles wide. Colluvial sediments, tuff breccia, fine pumiceous rhyolite tuffs, and rhyolite flows totalling more than 1500 feet were deposited in sequence in the moat. Similar deposits formed outside the caldera. K-Ar ages indicate that the period of ash-flow eruption and caldera formation is much less than a million years.

The Rhyolite Canyon sheet was block faulted and tilted throughout the range after emplacement. South of the caldera a major horst block, 3-4 miles wide and radial to the caldera, was

uplifted resulting in formation of thick alluvial deposits. These

deposits were derived in large part from the Rhyolite Canyon sheet. C H A P T E R X

DISCUSSION

Many challenging questions have been raised in the course of this research, questions relating to the structure and development of the caldera, to the volcanic and structural history of the region,

and to the interrelation of ash-flow eruption, caldera formation, and

Basin and Range activity. These questions could serve as the basis

for years of volcanological, geochemical, structural, and geo-

chronological research in the area.

Prerequisite to the solution of critical questions about the

caldera structure and history would be detailed mapping in and

immediately around the caldera, to a scale of about 1:15, 000. In

the course of this work, much information would be gained on the

nature of rock units in the caldera- the welded tuffs, moat deposits,

and monzonite. Questions such as the origin of the tuff breccia and

tuffaceous sediments, and whether or not caldera fill is exposed

could be answered. Source vents for the moat rhyolites could be

identified. Information on the nature of the monzonite pluton could

be obtained and the laccolithic hypothesis evaluated. This work would

lead to a better understanding of calderas in general.

135 136

When a composite section of Rhyolite Canyon tuff in the caldera is established, the problem of correlating this section with that in the type area (in the Monument) could be tackled. This problem may require special petrographic, chemical, or isotopic techniques. Relative percentage of phenocrysts will provide a general correlation. Chemistry should be sufficiently diagnostic to correlate all members. Magnetic correlation is not feasible in the field with present instrumentation because of relatively low magnetite content and may not be feasible even with laboratory instrumentation because of the ubiquitous heating by the underlying pluton and the low

dispersion in iron content between flows.

The caldera would be an ideal subject for geophysical study.

Geophysics could aid considerably in determining the subsurface

structure of the caldera and underlying pluton. Gravity surveying

would be quite effective because of the density difference (tuffs are

between 2. 0 and 2. 5 gm /cc and the monzonite between 2. 7 and 2. 9

gm/cc). Magnetic surveys would also be very effective in view of

the difference in iron content (tuffs contain about 2 percent FegOg

and a trace of FeO; the monzonite contains 4 percent FezOg and

3 percent FeO).

The petrochemical evolution of rhyolites in the caldera

sequence and the relation of the monzonite and andesites to the 137 rhyolites are other challenging problems. These bear on the question of the petrogenesis of the ash flows and monzonite. Is the monzonite a residual magma or a separate differentiate of a deep- seated magma? The fact that rhyolite erupted after the monzonite was emplaced favors the latter possibility. Geochemical studies would help answer questions like these. Sr®^/Sr86 data would provide evidence of the source of the magma, whether from the crust or mantle, and contamination of the magma. Sr analyses will require special techniques for the rhyolites, however, because of their extremely low Sr content.

The chem istry of the rhyolites in the caldera sequence is very interesting. With the exception of about 2 percent FegOg, the rhyolites are essentially composed SiOg, AI2 O3 , N a 2 0 , and K 2 O, true candidates for "petrogeny's residua system" of Bowen.

Other questions are in regard to the relation of the caldera

sequence to other volcanics in the Chiricahua volcanic field. The

underlying ash-flow sheet (Eagle Cliffs member of the Cave Creek

formation and lower rhyolite in Horseshoe Canyon) shows petrographic

sim ilarities with the Rhyolite Canyon tuffs and the K-Ar ages are in­

distinguishable. Is there a genetic relationship? Where is the source

caldera of the underlying ash-flow sheet? Actually it may be in the

Chiricahua Mountains. There could even be another caldera 138 associated with the Rhyolite Canyon Formation, obscured by later volcanism, tectonism, and the Turkey Creek caldera.

Proceeding now from problems of the Chiricahua volcanic field to the great problems of the Basin and Range, what role have

ash-flow eruption and caldera formation played in the development

of the Basin and Range? Were the tectonic adjustments which took

place throughout the Chiricahua range between the eruption of the

two ash-flow sheets the cause or the effect or were they entirely

independent of ash-flow eruptions? Was the formation of the present

basin and range structure, which the author has suggested followed

emplacement of the Rhyolite Canyon sheet, a consequence of ash-flow

eruption? Does the presence of the Turkey Creek caldera in the

Chiricahua range have anything to do with that being a range-block

rather than a basin-block? Questions such as these indicate the

challenge of ash-flow research in the Basin and Range. C H A P T E R XI

CONCLUSIONS

This research has shown that studies of volcanic rocks (and in particular, ash flows) can be very fruitful in the Mexican Highland and Sonoran Desert sections of the Basin and Range when due consid­ eration is given to the problem of correlation and to the nature of igneous and structural activity associated with ash-flow eruption.

It has been shown that a specific geologic problem, such as the

identification of the source caldera of the Rhyolite Canyon tuff, can be solved by the collection of field and laboratory data relating to

that problem, without the necessity of working out the detailed

geology of the area involved.

Results of this research have shown that ash-flow and

caldera studies in different geologic environments can greatly

advance our understanding of them. The present research has already

verified that intrusion of magma was associated with resurgent

doming of the caldera and that this intrusion was not confined to the

dome but also extended into the ring-fracture zone.

Finally, results of this research have indicated a close

relation between the eruption of major ash-flow sheets, caldera

139 140 formation, and basin and range tectonism. Further definition of this relationship is a challenge to volcanology in the Basin and Range

P r o v in c e . A P P E N D IX I

DETAILS OF REGIONAL RECONNAISSANCE

INVESTIGATIONS

This appendix describes the results of examinations of the

Tertiary rhyolite sequence in key area neighboring the Chiricahua

Mountains. Locations of these areas are shown in Figure 5.

Central Peloncillo Mts. --W eatherby Canyon (Area 1)

The W eatherby Canyon ignimbrite was described by

Gillerman (1958) as a sequence of interbedded rhyolite and subordi­ nate trachyte welded tuffs that are exposed over about 10 s q u a r e miles in the vicinity of W eatherby Canyon.

The author examined the sequence in the vicinity and immediately south of W eatherby Canyon, 1 particularly below 1117

Peak. A rhyolite tuff from near the base of the section is petro- graphically sim ilar to the Rhyolite Canyon tuff but exhibits slightly different coloring and weathering characteristics and is overlain

1. Sections 25, 34, 35, and 36, T. 26S. , R.21W. , and sec. 20, T. 26S. , R. 20W.

141 142 by biotite-bearing rhyolites. Rhyolites with biotite are almost exclusively pre-Rhyolite Canyon in age and these particular ones look like tuffs of probable pre-Rhyolite Canyon age composing the Eagle Cliffs member of the Cave Creek Formation (Figure 12) in the Cave Creek area. In spite of this evidence indicating a pre-Rhyolite Canyon age for the W eatherby Canyon ignimbrite, because of petrographic sim ilarity to the Rhyolite Canyon tuff a K-Ar analysis was made. The resultant age of 26. 3 +0. 8 m. y.

(DM-5-67, Appendix II) is close enough to the age of the Rhyolite

Canyon tuff (24. 9 +0. 6m. y. ) so that it doesn't positively decide one way or the other.

The biotite-bearingrhyolite mentioned above is found at the top of 1117 Peak. It contains up to 2 percent phenocrysts of biotite and sanidine. Biotite is 1 to 2 mm in size and sanidine, up to 5 mm in size.

Gillerman estimated total thickness of the Weatherby

Canyon ignimbrite as 3000 feet, assuming a continuous east-west

section. The author noted faulting in the section and suggests that the total thickness probably does not exceed 2000 feet.

Southern Peloncillo Mts.

Little is known of rhyolites in the Peloncillo Mountains beyond that indicated on Figure 5. Using this map as a base and 143 some additional information provided by Zeller (personal communi­ cation, 1967) the author examined the sequence in four widely spaced a r e a s :

2 South Antelope Pass (Area 2)

The section exposed in South Antelope Pass^ is described

in Table 15. Some of these units can be seen in Figure 49 . A thin layer of Rhyolite Canyon tuff is found capping the section im m ediate­

ly north of South Antelope Pass at an elevation of 6100 feet. This

rock is characterized by a total phenocryst amount and size com­

parable to that of lower members in the Monument.

Northeast of Black Mt. 4 (Area 3)

Light brownish gray rhyolite (? ) welded tuffs in this area

contain about 20 percent phenocrysts and 5 percent xenoliths.

Phenocrysts are mostly less than 2 mm and consist of quartz, san-

idine, plagioclase and biotite. Quartz exceeds feldspar and sanidine

is more abundant than plagioclase. Xenoliths do not exceed about

5 mm. These rocks may correlate with the biotite-bearing rhyolite

2. See footnote 2, Chapter III. Location of area studied: sec's 22, 23, 26, and 27, T. 28S. , R. 21W.

3. SW1/4, SE1/4, sec. 23, T.28S. , R. 21W.

4. Sec 33, T.29S. , R. 20W. 144

TABLE 15. - -Estimated Section at South Antelope Pass

U nit* Description F e e t* *

R h y o lite Light brownish gray; 12 % phenocrysts (up to 20 C anyon 2 mm); chatoyant sanidine and quartz in F m . about equal amounts; sparse iron oxide. Top e ro d e d .

air fall tuff Light brown. 110 or volcanic s a n d s to n e

rh y o lite Moderately welded, forming good columns; 300 w e ld ed pale-brown with black vitrophyre and under­ tu ff lying 50-foot thick light pink ashy zone; 25 to 35% phenocrysts of feldspar (up to 5 mm), quartz, and biotite (up to 3 mm) with feldspar 3 times more abundant than quartz; sanidine is chatoyant; trace of plagioclase; few inclu­ sions. Unit B in Figure 49 .

lith ic W eathers with appearance of thick bedding; 200 rh y o lite yellowish-gray to grayish orange pink ground- w e ld ed mass; grayish purple angular volcanic frag­ tu ff ments (around 1 cm) amount to 10-30% of the rock, increasing toward the base where they are up to 6 feet in size. Unit A in Figure 49 .

rh y o lite Laminated flows forming broad, irregular 150 flow columns; light brownish gray with pink specks; localspheroidal and bottryoidal zones. Base c o v e r e d .

* Formation or rock type. Classification is based on megascopic examination.

** 4^30 percent. 145

FIGURE 4 9 . -----View of overlapping ash flows in South Antelope Pass , Peloncillo Mountains. Lettered units are described in Table 15. Total relief: about 700 feet. in the Weatherby Canyon area described above.

Post Office Canyon to Woodchoppers Spring^ (Area 4)

The section below Owl Peak is described in Table 16.

Another welded tuff near the base of this section but not described above is very similar to one in the Weatherby Canyon and Black

Mountain areas (areas 1 and 3, respectively). This rock is slightly less porphyritic, however, containing about 10 percent phenocrysts.

5. Sec. 31, T.29S. , R. 21W. ; sec's 5-8, 17-20, 29-32, T.30S. , R.21W.; sec's 25, 36, T. 30S. , R. 22W. ; sec. 6, T.31S. , R. 21W. ; sec. 1, T.31S. , R. 22W. 146

TABLE 16. - -Estim ated Section Below Owl Peak

R o ck u n it* Description F e e t* *

rh y o lite Densely welded with some vertical 350 w e ld ed joints; light brownish gray in upper part tu ff grading into pale red toward the base; inclusions up to 1 cm amount to less than 5 percent; 30—40 percent phenocrysts of feldspar and quartz (up to 4 mm) and biotite (2-3 mm); biotite is 2-5 percent of rock. Top eroded.

rh y o lite Partially welded forming subvertical 900 w e ld ed columns; very pale orange to pinkish tu ff gray with black vitrophyre base; xeno- lith content and size increases from 10 percent (1 cm size) fragments in upper parts to 30 percent (around 8 cm) frag­ ments toward the base; 15-20 percent phenocrysts of quartz and feldspar (up to 4 mm) and biotite (to 1 mm) , biotite amounts to 1 percent.

rh y o lite M ulti-colored, predominantly white and 200 w elded pinkish gray; 10 percent phenocrysts of tu ff sanidine, quartz and biotite, biotite isup to 2 mm in size. Base covered.

* Classification on megascopic characteristics

** 30 percent.

\ 147 Cottonwood Creek to Clanton Draw (Area 5)

A traverse was made along the Geronimo Trail east across the range. ^ Finely porphyritic and glassy rtyolite(?) flows pre­ dominate in this area, although most of the area was mapped as welded tuff by Wrucke and Bromfield (1961). These rocks may be cliff-forming brownish rocks or thinly laminated light bluish gray to black rocks. Possible source vents are indicated by persistent vertical flow lamination.

Per ilia Mts. --College Peaks and Poverty Flat? (Area 6)

At least six flow units are found in this area. They range from densely welded pale brown cliff-forming tuffs, several hundred feet thick, to m oderately and poorly welded pinkish gray and light gray tuffs that may not be more than a few tens of feet thick. The prominent cliffs of North College and South College Peaks are made up of the form er. These rocks contain 10-15 percent phenocrysts and less than 5 percent xenoliths. Phenocrysts are quartz, sanidine, plagioclase, and biotite. Sanidine is about 4 times as abundant as plagioclase. Biotite amounts to 1 to 2 percent.

6. From sections 22 and 23, T.22S. , R. 32E. in Arizona to sec. 17, T. 32S. , R. 21W. , in New Mexico.

7. NE1/4, sec. 9, T.23S. , R. 29E. ; and SW cor. , SEl/4, NE1/4, sec. 35, T.22S. , R. 28E. 148

Swisshelm Mountains (Area 7)

A variety of tuffs are found in the Swisshelm M ountains, including: Rhyolite Canyon tuff, biotite-bearing tuffs, lithic welded tuffs, and w ater-laid tuffs or mudflows. The Rhyolite Canyon tuff is exposed along the road cut entering the range from the northeast. ®

Here it contains 25 percent phenocrysts of quartz and sanidine, 2 to

3 mm in size. Magnetite is relatively abundant. This composition is consistent with correlation with an upper member of the formation.

Sulphur Hills (Area 8)

Gilluly (1956) described 540 feet of rhyolite flows and tuffs, including welded tuffs, in the Pearce volcanics. The author examined these rocks in the Sulphur Hills. 9 No welded tuffs were found. The numerous other hills in the Sulphur Springs Valley were not examined.

8. In sec. 31, T.19S. , R. 28E.

9. NE1/4, sec. 7, T. 17S. , R. 26E . APPENDIX II

K-Ar AGE DETERMINATION

This appendix includes a brief description of the procedure used for sample preparation and K-Ar analysis, * tabulated analytical and sample data, discussions of discordant ages, and a determination of the tim e-span of ash-flow eruption and caldera formation.

Sample Preparation

M ineral separation was performed by standard methods.

Initial preparation involved crushing in jaw and roller crushers, pulverizing, sieving, and washing. M ineral separation was achieved using magnetic separators, a vibrating table, and heavy liquids. In some samples ultrasonic treatm ent proved to be very effective in removing sericitized feldspar, especially in samples DM-1-68 and DM-2-68. Prior to treatment the composition was estimated

at about 85 percent clear sanidine and 15 percent cloudy sanidine.

After 90 minutes in the ultrasonic tank followed by sieving, the

coarser fraction was 95 percent clear and the remainder cloudy.

1. The procedure for sample preparation and K-Ar analysis followed in this laboratory has previously been described by Livingston et al (1967).

149 150

Composition of the final separate was determined by counting in an immersion oil of index intermediate between that of quartz and

sanidine. Final concentrations exceeded 95 percent.

Analytical Procedure

Potassium analyses were performed on a modified Perkin-

Elmer flame photometer. Samples were digested with HF, H 2SO4 ,

and HC1. Na was used as a buffer and Li was used as an internal

standard. Precision in this laboratory is indicated by a standard

deviation of 4^0 . 56 percent on duplicate analyses of 150 samples.

The accuracy indicated by replicate analyses of 10 interlaboratory

standards is + 1. 54 percent (s.d. ) of the amount contained.

Argon analyses were made by the isotope dilution method on

a Nier type 6 -inch, 60° mass spectrom eter. Highly enriched (99.9%)

Ar^B was used as a diluent. Gases were purified with synthetic zeo­

lite, hot copper oxide and titanium sponge. Mass spectrom etric anal­

ysis was by the dynamic mode. Precision of + 1.74 percent (s. d. )

has been been obtained on seven duplicate analyses and an accuracy

of +0. 5 percent (s.d. ) for three interlaboratory comparisons.

Analytical and Sample Data

Analytical data are tabulated in Table 17, sample identifi­

cation and location in Table 18, and sample petrography in Table 19. TABLE 17. --K-Ar Analytical Data

Laboratory Sample Mineral Mesh K Ar4 ® rad. Ar4 ® atm. Apparent age Notes sam ple no. . prep. size % x l0 “12m /g m % m. y.

DM-1-67 biotite -48. +65 6.82 340.5 73.6 27.9 + 2.0 1 336.5 30.8 27.6 +0.8 2

DM -2-67 - sanidine -48. +65 6.75 301.8 14.2 25.0 +0.8

DM -3-67 - sanidine -48. +65 6.31 280.5 1.5 24.9 + 0.7

DM -4-67 A sanidine -48, +65 11.80 450.0 6 .7 21.4 +0.6 3 B sanidine -150, +300 11. 19 451.7 7.6 21.3 + 1.3 3,4

DM -5-67 - sanidine -48, +65 6.72 316.2 1.7 26.3 +0.8 5

DM -6-67 A biotite -65, +100 7.20 323. 2 36.4 25. 1+4.4 6 B biotite -100, +150 7.04 323.2 4 1 .6 25.7 +0.8

DM -1-68 - sanidine -65, +150 7.42 325.3 8. 7 24.5 + 1.2 6

DM -2-68 - sanidine -100, +150 8.66 448. 3 11.4 28.9 +1.9 6

DM -3-68 - biotite -100, +150 7.48 331.8 32.3 24.8 + 0.7

P E D -12-62 A sanidine -20. +35 5.75 165.5 4 0 .8 16. 2 + 1.6 7 273. 7 57. 8 26.6 + 1.1 1 B sanidine -65. +100 5.90 254.9 2 .2 24. 2 + 0.7 8

Notes: 1. Probable leak in fusion system indicated by unusually high atm. Ar (moles/gm). 2. Pre-baked for 100 hours at 100*C. 3. See text. 4. Treated in 10% HF for 15 min. ; washed, sonically cleaned and sieved. 5. Probable error may be somewhat higher because of lower sensitivity (moles/millivolt) by a factor of 5, which is a consequence of a short half-life during dynamic analysis. 6. Large error in result due to high Ar3 background during dynamic analysis.

7. Damon et al, 1962. See text for discussion. 151 8. Pre-baked for 50 hours at 100°C. TABLE 18. --K-Ar Sample Identification and Location

Laboratory no. Formation, member, Position in Reference (Field no. ) (map unit*) mem. or unit Location**

DM-1-67 Faraway Ranch F m ., mem. 7, Fernandez and near top of Monument; west wall of Newton Canyon; 20 ft. below ridge; SW1/4, (70) (lower rhyolites) Enlows, 1966 member NWT/4, sec. 35, T. 16S., R. 29E.

DM-2-67 Rhyolite Canyon F m ., mem. 2, Enlows, 1955 middle of Monument; 100-200#. above bottom of Little Jesse James Canyon (71) (Rhyolite Canyon F m .) member in west wall; center NW1/4, sec. 1, T. 17S. , R. 29E. DM-3-67 Rhyolite Canyon Fm. , mem. 8, Enlows, 1955 Monument; along Sugarloaf trail; SE1/4, SE1/4, sec. 24, T. 16S., (69) (Rhyolite Canyon F m .) R.29-1/2E.

DM-4-67 Cave Creek F m ., Rancho Raydon, 1952 50 ft. above Cave Creek; SW cor. NE1/4, SE l/4, sec. 35, T. 17S. , R. 31E. at (149) Risco mem. , (lower rhyolites) base of mem. 5300-ft, elevation.

DM-5-67 Weatherby Canyon Ignimbrite, Gillerman. near base New M ex., Hidalgo Co. , Peloncillo Mts. ; south edge of SW1/4, (150) — — — — — (— — — — *) 1958 NWT/4, sec. 36, T. 26S., R.21W. ; at about 5000 ft. elevation, above pale yellowish gray ash beds, 100 ft. above base of 5785 ft. peak on its east side.

DM-6-67 Cave Creek Fm. , Eagle Cliffs Raydon, 1952 near base Cave Creek; up NNW trending canyon off Cave Creek, 1/2 mi. (418) mem., (lower rhyolites) of mem. west of junction of Cave Creek and South Fork; below cliffs on east wall, 3/8 ml. up canyon; T. 18S. , R.31E.

DM-1-68 -----, , (lower rhyolites) This work upper part Horseshoe Canyon; 75 ft. above bottom of Pothole Canyon; NE cor. (556) of unit of NE1/4, NE1/4, sec. 16, T. 19S., R.31E.

DM-2-68 Faraway Ranch Fm. , mem. 3, Fernandez and Monument; west side of Erickson Ridge; north side of NNEtrending (543) (lower rhyolites) Enlows, 1966 draw in ctr. NE1/4, NW1/4, sec. 2, T. 17S., R.29E.

DM-3-68 This work Rock Canyon; in road cut at ctr. NW1/4, SE l/4, sec. 9, T. 18S., (554) R.29E.

PED-12-62 Rhyolite Canyon F m ., mem. 6 Enlows, 1955 near top of Monument; Massai Point; SW1/4, NE1/4, sec. 30, T. 16S., R.30E. (Rhyolite Canyon F m .) member

* Plate 1.

** Unless otherwise indicated, locations arc in Cochise County. Arizona. 152

I TABLE 19. - -Petrography of K-Ar Analyzed Samples

Laboratory no. Rock type Macroscopic Microscopic

DM -1-67 rhyodacite Banded or blocky at scale of 5mm; 30% phenocrysts (0. 5-2.0 mm); 25% andesine, less flow pale-brown and very light gray; than 5 % san; fresh bio laths (to 1 mm) amount to somewhat ashy or crumbly 2% of the rock; minor hbl and mag DM- 2 -6 7 rhyolite Light brownish gray, hard; with an 8% phenocrysts (0. 2 -1 .4 mm); san and qtz in about welded tuff occasional chatoyant san phenocryst equal amounts; san is fresh and Carlsbad twinned; qtz has rounded corners; less than 1% bio DM-3-67 rhyolite Grayish red with chatoyant sanidine 35% phenocrysts (0. 2-2.0 mm); san slightly more welded tuff and highly flattened, short, crystal­ abundant than quartz; few percent mag (0.2 mm) lized lenses with associated hem DM-4 -6 7 rhyolite Very pale orange; moderate pale Less than 5% phenocrysts (1.0-1.5 mm), predomi­ welded tuff brown patches, probably remnants nantly kaolinized san and subordinate small, highly of pumice, are seen in bedding corroded qtz; sparse mag and hem; no shards seen plane DM-5-67 rhyolite Light brownish gray with pinkish 13% phenocrysts (up to 2 mm) of slightly altered welded tuff tinge and fractured san, euhedral qtz, and sparse mag DM -6-67 rhyolite Light brownish gray with 20% very 20% phenocrysts (up to 2 mm); san mostly kaolin­ welded tuff pale orange lenses; densely welded ized; qtz is rounded; bio laths (1-2 mm) amount to 1-2% and are partially embayed, fractured, and associated with mag DM-1-68 rhyolite Pale red with grayish red lenses; 30% phenocrysts (up to 2 mm), predominantly san welded tuff densely welded and plag with minor qtz; slightly altered san is more than half of feldspar; 2-3% (3 mm) reddened and fractured bio laths; mag and clinopyxafter bio DM- 2 -68 rhyolite Grayish pink; poorly welded 20% phenocrysts (up to 2 mm); slightly altered and welded tuff fractured san in excess of quartz; trace plag; less than 5% bio; trace mag DM -3-68 porphyritic Grayish red and light gray, banded Up to 15% phenocrysts (up to 3 mm); fresh K-feldspar rhyolite dike at scale of 5 mm width and qtz; few percent bio laths (nominally 1/2 mm) PED-12-62 rhyolite Light gray, poorly welded 20% phenocrysts (up to 3 mm); san is fractured to welded tuff 0. 1 mm, alteration to clay follows fractures;

abundant very small mag in matrix 153 154

Discussion of the Analyses of PED-12-62

An age of 16. 2 4^1. 6 m. y. was previously reported on sample PED-12-62 (Damon et al, 1962, p. 25), collected from member 6 of the Rhyolite Canyon Formation. Since this age was in discordance with the author's results of 24. 9 +0. 7 and 25. 0 +0. 8 m. y. on members above and below it (DM-3-67 and DM-2-67, respectively) the sample was re-examined. It was found that san- idine fragments used for the analysis (0.4 to 0. 7 mm; 20 to 35 mesh) were highly fractured down to a scale of 0. 1 mm. Clay m inerals accompanied these fractures and it was suspected that this was the reason for the young age. To verify this, a split of the sample was pulverized to a sm aller size and the size fraction, 0. 15 to 0. 20 mm,

(65 to 100 mesh) separated. It was assumed that in this size range most of the clay would be exposed in crushing and could easily be re ­ moved by washing and Bonification. An age of 24. 2 +0. 7 m. y. ob­ tained on this split is in satisfactory agreement with the author's other results.

It is noted that this sample was collected from the upper part of the 900-foot thick ash flow. Decomposition of sanidine was probably caused by reaction with volatiles rising during cooling and

devitrification. The author has noted that the phenocrysts are best

preserved in the basal portions of ash flows. Nevertheless, the 155 re-analysis of PED-12-62 has shown that even highly decomposed sanidine gives concordant results if properly prepared.

Discussion of the Discordant Age of DM-4-67

The age obtained on DM-4-67 from the Rancho Risco m em ­ ber of the Cave Creek formation is in discordance with an age obtained on an overlying member and with field evidence of a pre-Rhyolite

Canyon age. The mean age obtained on DM-4-67 from duplicate analyses is 21.3 +0. 7 m. y. The age of an overlying member is

25. 7 +0. 8 m. y. (DM-6-67). Field evidence indicates that the entire

sequence is pre-Rhyolite Canyon in age.

Difficulty was encountered in obtaining a su itable sample

from the base of the Cave Creek formation because of alteration and

the fine texture of the rocks. DM-4-67 was collected and anlyzed •

even though sanidine showed signs of considerable alteration. P er­

haps influenced by the success in the re-analysis of PED-12-62, the

author thought that the sanidine from this sample could be cleaned,

even though it was fractured and altered down to less than 0. 1 mm.

- y Discordance of the age is considered to be due to sample

alteration. This conclusion is based upon: (1) the extensive m icro­

scopic fracturing, decomposition, and brownish coloring of the

sanidine, (2) the abnormally high K content of the sanidine, more

than 11 percent, and (3) the presence of a nearby intrusive. The 156 small intrusive body is located about 1000 feet from where the sam ­ ple was collected, according to Cooper's (1959) map. Its presence was not known when the sample was collected because it is not shown on Raydon's (1952) map which was used as a base at that time. APPENDIX III

PHENOCRYST ANALYSIS

This appendix describes the results of analyses of phenocryst abundance and size-distribution in individual members of the Rhyolite Canyon Formation. These analyses were made to evaluate possible methods for quick correlation of members in field mapping.

For these analyses, rock slabs were stained to facilitate distinguishing between sanidine and quartz in rock slabs. Counting was done with a low powered binocular microscope. Staining pro­ cedure is sim ilar to that described by W illiams (I960) for volcanic rocks. Staining of K-feldspar in rhyolite poses a problem in that

it is difficult to obtain appreciable contrast between the stained feldspar and the groundmass. The feldspar, containing around

7 percent K stains much the same as the groundmass which contains

slightly less K, around 4 percent, but is more easily etched. For

fresh sanidine phenocrysts, best results were obtained by etching

for 15 seconds in 50 percent HF. For partially kaolinized, silvery

white sanidine, etching for as little as 2 seconds was sufficient.

157 158

Partially welded and nonwelded samples were impregnated with xylene and burgundy pitch in a 5:1 mixture by volume. The

slabs and mixture were placed in a beaker and the xylene was boiled

off. After the slab was cooled, it was polished and stained.

Phenocryst Size and Percentage

Since total phenocryst percentage and size generally

increase upward in the sequence (Table 20), they are useful in deter­

mining relative position in the sequence. Vertical and lateral sort­

ing effects are too great, however, even over distances of a couple

of miles, for these to be reliable in distinguishing members.

Relative Phenocryst Abundance

The relative abundance of quartz, sanidine, and magnetite

phenocrysts show significant variation in the Monument section to

be able to distinguish between the members (Table 20 and Figure 50).

For correlation over distances of several miles, however, the effects

of sorting must again be taken into consideration. With increasing

distance from the source vents, the relative percentage of magnetite

would decrease relative to quartz and sanidine due to its higher den­

sity while sanidine would increase relative to quartz because of its

higher degree of fragmentation. Some degree of correlation should

be possible if these effects are taken into consideration. As an 159

TABLE 20. --Phenocryst Abundance in Welded Zones of Rhyolite Canyon Formation

F ie ld M e m ­ Position in member % of % of phenocrysts1t* no. b e r or location ro c k * san Q san/Q m a g

M o n u m en t 69 8 lower half 17. 8 60. 5 2 8 .5 6. 6 4. 3 264 6 300 ft. from top 25. 1 51. 7 45. 6 1 .5 2. 7 268 4 35 ft. above base 15. 7 54.3 40. 1 2. 8 2. 5 270 3 30 ft. from top 22. 8 57. 7 3 5 .4 6. 0 1 .7 71 2 m id d le 12. 2 55. 0 4 1 .7 2. 0 1. 7

Horseshoe Canyon 467 31. 2 6 1 .5 30. 9 3. 0 4.6

C a ld e ra

161 top Chiricahua Pk. 31. 0 5 9 .7 3 0 .3 3 .3 6. 7 345 - base Chiricahua Pk. 36. 5 60. 5 34. 2 2.6 3.3 495 - Rock Peak 13. 5 54. 6 40. 1 1.3 3 ,9

* Based on more than 600 counts. Based on more than 300 counts of phenocrysts. 160 W////////k

EXPLANATION Numbers indicate member number in the Monument m a g n e tite HC Horseshoe Canyon q u a r tz CPT Top of Chiricahua Peak sa n id in e CPB Base of Chiricahua Peak □ o r q u a rtz RP R ock P e a k m s a n id in e

FIGURE 50.—Relative percentages of phenocrysts in Rhyolite Canyon welded tuffs 161 example, consider a sample that contains 2% magnetite and is closer to the source than the reference section in the Monument. From the data in Table 20 it is obvious that it could only correlate with m em bers

2 or 3 which have less than 2% magnetite in the Monument. With such considerations it is seen that of the samples analyzed outside of the Monument, the Rock Peak sample could correlate with members

2, 3, o r 4; the base of Chiricahua Peak with member 3; and the top of Chiricahua Peak and Horseshoe Canyon with members 3 or 8.

This method appears promising but needs further evaluation, particularly with regard to variations within individual m em bers.

Chemical data could be used to verify the correlations. This method might also be used to determine general location of the source vents in the caldera.

Quartz Size-Distribution

It was though that perhaps the size-distribution of pheno- crysts might reflect eruptive conditions unique to certain ash flows so that such a distribution could serve as the basis for correlation.

In this analysis, quartz was used rather than sanidine because it is much less fractured and its size more easily measured in view of the euhedral to subhedral shape of crystals. Size -distributions were obtained by measuring the area of quartz crystals in the stained

slab with a binocular microscope containing a grid at the focal plane 162 of the eyepiece. Areas were tallied up within the size-ranges and the histogram normalized to a total of 100 percent, excluding sizes below 1/2 mm. In the reference section, members maybe distin­ guished by their distributions, with successively younger ash flows having broader distributions and a larger maximum crystal size

(Figure 51). In correlating, however, the effects of sorting must again be taken into consideration. Some suggestion of correlation may be seen for samples analyzed outside of the Monument (Figure

52 ), but obviously the data is not definitive at this stage.

Discussion of the Origin of Member 5

Member 5 of the Rhyolite Canyon Formation is interesting because it is only 30 inches thick yet contains a 16-inch thick com­ pact, densely welded zone (Figure 53 ). Enlows concluded that the dense welding and flattening of pumice blocks in the base of member

5 is a consequence of: (1) deposition of member 6 before member 5 had cooled appreciably, and (2) the insulating effect of the ash in between the welded zones of members 6 and 5.

Quartz size-distributions for various zones in the section

in Figure 53 show no pronounced discontinuity in the section from

the base of member 5 to the base of member 6 (Figure 54).

Member 5 is only slightly finer grained than member 6 on the 163

4 0 - 4 , b o tto m (268), 50 30-

2 0 -

10

0 r 1 2 m m

m m

40 2, m id d le (71), 40 30 % 20

10 0 tin. 1 2 m m

FIGURE 51 . - -Quartz size-distribution in welded zones of the Rhyolite Canyon Formation in the Monument. Ordinate is percentage contribution of size-range to tokal area of quartz, normalized to 100 percent above 1/2 mm. Abcissa is square root of crystal area. Label includes member number and relative position in member and, in lower row, field sample number and total area of quartz counted in mm^. Area of slab counted is about 8 square centimeters. 164

Horseshoe Can. 40- Chiricahua Pk. (161). 75 30-

2 0 - 104 0 1 2 m m m m

Horseshoe Can. Sentinel Pk.

m m m m

Horseshoe Can. P r i c e C an. 40 A (194), 40 30

20 -

10 -

0 1 2 m m m m

FIGURE 52.--Quartz size-distribution in welded zones of the Rhyolite Canyon Formation outside the Monument. Coordinates are the same as in Figure 51 . Label includes location and, in lower row, field sample number and total are of quartz counted in mm^. Area of slab counted is around 8 cm^. 165

Field Thick. Description Member no. no.

Firmly welded vitrophyre; mem 6 1 - 260 grayish red 261 6 in. Incoherent tuff, very light gray 262 Somewhat coherent tuff, light 8 in. gray; separated above by cm- thick coarse crystal tuff m e m . 5 Black to dark gray, firmly welded with flattened pinkish- '—v-* 16 in 8ray to very light gray lenses, 5-6 cm long in a dark, aphan- itic groundmass

Light gray porous tuff; upper , . 1 m 6 m • contact covered

FIGURE 53. - -Section through member 5 in the Monument. In road cut at China Boy in Bonita Canyon. Modified after Enlows, 1955, p. 1228.

whole. The sim ilarities in overall lithology and size-distribution are consistent with member 5 being a "subflow" (Smith, I960, p. 811) of m e m b e r 6.

A centim eter-thick layer between the upper two zones of member 5 is composed mainly of medium sized crystals. It appears that there was a tim e-break between emplacement of members 5 and 6 long enough to allow sifting of the tuff to generate this layer. 166

6, near top 5 0 - (264) 4 0 “ % 3 0 -

20 " 5 101 0 tm I 1 2 3 m m

m m

m m

m m

m m

F IG U R E 54 . --Quartz size-distribution in zones of Rhyolite Canyon Formation, members 5 and 6. Coordinates are the same as in Figure 51. Label includes member number, and in lower row, field sample number. Sample locations are indicated in Figure 53 , with the exception of sample 264. Area of slab counted is about 8 cm^. APPENDIX IV

X-RAY FLUORESCENCE ANALYSIS

Included in this appendix are (1) a description of the procedure for preparation of whole rock samples for X-ray- fluorescence analysis, (2) X-ray fluorescence techniques, and

(3) sample identification and location data.

Sample Preparation

Because of the low Ca content, it was anticipated that rhyolites of the caldera sequence would have low Sr content.

Consequently, special care was exercised in the preparation of samples to avoid Sr contamination from weathering along fractures and from xenoliths. To insure representative composition, a sample of 10-lb. weight was prepared. * After the sample was crushed to sm aller than 1 cm. , m aterial sm aller than 9 mesh was removed.

Next, the sample was split several times and a split weighing

about 75 grams retained. Any fragments showing evidence of

xenolithic m aterial or weathering were removed. Up to 15% of

the rock had to be removed at this stage. Finally, the m aterial

1. Sample 322 is an exception; it is a hand sample.

167 U>8 wns pulverized in a Pitehford blender mill to finer than about 100 mesh. The powdered sample was then pressed into pellets.

Duplicate Rb and Sr analyses on samples with and without hand-picking surprisingly did not show significant differences.

When hand-picked, samples 400, 595, 596, and 592 gave the same

Rb content within 1. 5 O' (s.d. of the analysis). Sr was also quite

close to the same except one sample which was 5(f higher, just

the opposite deviation one would expect if hand-picking was effec­

tive in removing a source of Sr. These results indicate that Rb

and Sr content in the rock and xenoliths are essentially the same

and that chemical contamination is negligible, a surprising result

since many of the fragments look like andesite and limestone.

Samples for major oxide determination were prepared in

the same manner as for Rb and Sr.

Analytical Techniques

Details of instrumentation and procedure may be found in

Eastwood (1970) and, in part, in Damon et al (1967 and 1968).

Analyses were made on a. Norelco vacuum X-ray spectrograph

using a Mo or Cr target and a gypsum, LiFg, or EDDT analyzing

c r y s ta l.

Accuracy of Rb and Sr analyses are of a different order

of magnitude than the major oxide analyses. The form er are 169 generally better than 3% of the amount contained except for concen­ trations of less than 20 ppm. The latter are only accurate to about

5%.

Oxide concentrations were determined using a modified least squares calibration line through wet chemical analyses re­ ported by Volborth (1963). The scatter in data on which the cal­ ibration line depended was used to calculate an rm s deviation in the vicinity of the sample value. This is the error reported in the data (Table 13).

Sample Identification and Location

The stratigraphic identification and geographic location

of samples analyzed by X-ray fluorescence are given in Table 21. 170

TABLE 21. - -Stratigraphic Identification and Location of Samples Analyzed for Whole Rock K, Rb, and Sr

F ie ld M ap Position in L o c a tio n * * no. u n it* u n it

568 m o a t near base of sector I; south of entrance rh y o lite s uppermost flow to Rock Can. ; south edge of NW1/4, NE1/4, sec. 1, T. IBS., R.28E.

628 m o a t near base of sector I; south wall of Pin­ rh y o lite s uppermost flow ery Can. ; 500 ft. south of SW cor, SE1/4, SE1/4, sec. 12, T. 17S. , R. 29E.

322 porphyritic e ro d e d to p sector IV; top of Stanford rh y o lite s Pk. ; SWcor, SE1/4, SE1/4, sec. 34, T. 18S., R. 29E.

400 R h y o lite n e a r to p sector II; top of Chiricahua C an. F m . P e a k

595 R h y o lite m e m b e r 8 Monument; Sugarloaf Mtn. ; C an. F m . SE1/4, SE1/4, sec. 24, T. 16S., R.29-1/2E.

596 Rhyolite upper part of Monument; base of Sugar- C an. F m . m e m b e r 6 loaf Mtn. at start of trail; SW1/4, SW1/4, SW1/4, sec. 19, T. 16S. , R. 29-1/2E.

592 Rhyolite 75 ft. above Monument; Bonita Can. ; C an. F m . base of mem. 6 China Boy; near center of sec. 24, T. 16S. , R. 29-1/2E.

613 R h y o lite top of mem. 4 Monument; north of Silver C an. F m . Spur Ranch; western half of NE1/4, SE1/4, sec. 26, T. 16S. , R.29E. at 5400-5800 ft. elevation 171

TABLE 21. - -(Continued)

F ie ld M ap P o s itio n L o c a tio n * * no. u n it* in u n it

597 R h y o lite b o tto m of same as 613 C an. F m . m e m b e r 4

602 R h y o lite to p of same as 613 C an. F m . m e m b e r 2

614 R h y o lite b o tto m of same as 613 C an. F m . m e m b e r 2

551 m o n z o n ite 400-800 ft. sector III; M orse Can. ; p o rp h y ry f r o m to p 2000 ft. east of SE cor. NE1/4, NE1/4, sec. 24, T. 18S., R. 29E. inroad cut, west wall of canyon

631 m o n z o n ite 400-800 ft. sector IV; south side of p o rp h y ry f r o m to p NW1/4, NE1/4, NE1/4, sec. 20, T. 18S. , R. 29E.

611 lo w e r F a r a w a y Monument; south side of r h y o lite s R an c h F m . state highway of NE side of Erickson Ridge; SW cor of SE1/4, SE1/4, sec. 27, T. 16S. , R. 29E.

* Geologic map, Plate 1.

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, 'S* *77P ' *-*822 ' ?' ■ H i m m * ~R F n > o li

Base from U.S. Geological Survey map of Chiricahua 30-minute quadrangle, 1917 TOPOGRAPHIC MAP OF THE CHIRICAHUA AND PEDREGOSA MOUNTAINS, ARIZONA SCALE 1:125 000

2 3

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Plate

EXPLANATION

ROCK UNITS IN CALDERA ROCK UNITS OUTSIDE OF CALDERA

> Ou a: < z Quaternary rocks > 5 Alluvium and olivine basalt. Alluvium includes \- unconsolidated gravel, sand and silt. Basalt < includes flows, tuffs, breccias, and cinder ZD cones; found mainly in southern part. O

Older alluvium Poorly consolidated, epiclastic, coarse-grained volcanic.sandstone and conglomerate. Includes Mesa Draw formation of Epis (1956) in the southwestern part.

ANGULAR UNCONFORMITY

Tmr Tru

Moat rhyolites Upper rhyolites Flow's, tuff breccia, tuffaceous sedim ents. air Tuff breccia, bedded tuffaceous deposits, and fall tuff, fine pumiceous tuffs. and flows flows. generally in that order of emplacement. generally in that order of emplacement. Includes quartz latite flows Essentially correlative with Tmr. T ri/

Silicic dikes Glassy to fine-grained; porphyrin range from Tmp to Tmr.

Tmp > (Z Monzonite porphyry Fine-grained b lot it e-hornblende monzonite with tr coarse andesine phenocrysts

•2 Tpr

Porphyritic rhyolites Flow or possibly welded tuff. Could be as young

Trc Trc

Rhyolite Canyon Formation Rhyolite Canyon Formation Rhyolite ash-flow tuffs. predominantly welded Rhyolite ash-flow tuffs . and commonly altered

ANGULAR UNCONFORMITY

Trl

Lower rhyolites Ash-flow tuffs, flows . air fall tuffs, arid volcanic sediments , in order of ciecreasing

ANGULAR UNCONFORMITY

PT

Pre-Tertiary rocks Lavas and breccias of intermediate composition; volcanic sediments. Includes rhyolite dikes, plug-like and sill-like bodies of Tertiary age and a small coarsely porphyritic stock at the entrance to Horseshoe Canyon, mapped by Cooper (1959) and Epis (1956). Also includes andesitic rocks of Tertiary - Cretaceous age mapped by previous workers.

Contact Dashed where approximately located

D Fault OasOea wiwre approximately located U. upthrown side: D, downthrow,, side

Strike and dip of beds

Strike of vertical beds

© Horizontal beds ,

Strike and dip of foliation

Strike of vertical foliation

Approximate topographic wall of Turkey Creek caldera

MAP SHOWING STRATIGRAPHIC-STRUCTURAL SUBDIVISIONS REFERRED TO IN TEXT

Subdivisions outside caldera are referred toas areas inside the caldera as sectors.

B 10000 Turkey Creek Pinery Creek 10r JO' Tmp Trc 6000 j TfC Trr.p Trip X i >1 eT Trl undiff ti 2000 ?

IOOOO Pinery Creek Tmr Trc . Tmr C a ve Creek Tmp V pT MAP SHOWING SOURCES OF Tmp Trc ’ GEOLOGIC DATA 6000 TT 1. Detailed and reconnaissance studies and mapping by the author, January 1967 through August 1968. Inter- Tertiary contacts are, with minor exceptions, from this work.

Reconnaissance mapping by Cooper Chiricahua Pk (1959). Intra-period contacts and Sentinel Pk some structural data are from Cooper's MAP SHOWING LOCATION OF MAP AREA map with minor modification. RELATIVE TO 15-MINUTE QUADRANGLES T^xjrc 2. Detailed study by Sabins (1957) whose Tmp Map area is cross-hatched. _ Jru _ map has been generalized. 6000 I LTrc f -.. 3. Detailed study by Enlows (1955) and Trl, Trc... Fernandez and Enlows (1966), whose ? f Drafting by Derwin Bell maps have been used without modifica- 2000 i- The University of Michigan

Base from U.S. Geological Survey map of Geology by D. K. Marjaniemi, 1967-1968 Chiricahua 30-minute quadrangle, 1917

TERTIARY GEOLOGIC MAP AND SECTIONS OF THE CHIRICAHUA AND PEDREGOSA MOUNTAINS, ARIZONA

SCALE 1:125 000 2______3

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